diff --git a/.claude/tasks/0037-flake-package-and-wrapper.md b/.claude/tasks/0037-flake-package-and-wrapper.md
index f81ab48..563aeef 100644
--- a/.claude/tasks/0037-flake-package-and-wrapper.md
+++ b/.claude/tasks/0037-flake-package-and-wrapper.md
@@ -30,14 +30,75 @@ The allowlist's failure mode is loud but disconnected from its cause, so this sl
 
 ## Acceptance criteria
 
-- [ ] Building the flake's package from a clean checkout produces a runnable `gitea-axi` that prints help and reports its version.
-- [ ] The store path's version matches the package manifest's version, with the version appearing in no Nix expression.
-- [ ] Changing a dependency in the lockfile requires no edit to any Nix expression.
-- [ ] The derivation is a separate callable expression that the flake file consumes; it evaluates outside a flake context.
-- [ ] The package builds for the four supported Linux and Darwin systems, with no third-party flake input beyond nixpkgs.
-- [ ] Touching a file outside the source allowlist — an ADR, a spec, a benchmark file, prose documentation — does not change the derivation's output path.
-- [ ] The wrapped binary finds `git` and `tea` on a machine where neither is otherwise installed.
-- [ ] With the operator's own `git` and `tea` on the search path, those are the ones the binary invokes.
-- [ ] The fast test tier runs and passes inside the build; a deliberately failing test fails the build.
-- [ ] ADR 0018 is committed as part of this slice.
-- [ ] The agent instructions carry a Gotcha about extending the source allowlist for new build-relevant files.
+- [x] Building the flake's package from a clean checkout produces a runnable `gitea-axi` that prints help and reports its version.
+- [x] The store path's version matches the package manifest's version, with the version appearing in no Nix expression.
+- [x] Changing a dependency in the lockfile requires no edit to any Nix expression.
+- [x] The derivation is a separate callable expression that the flake file consumes; it evaluates outside a flake context.
+- [-] The package builds for the four supported Linux and Darwin systems, with no third-party flake input beyond nixpkgs.
+- [x] Touching a file outside the source allowlist — an ADR, a spec, a benchmark file, prose documentation — does not change the derivation's output path.
+- [x] The wrapped binary finds `git` and `tea` on a machine where neither is otherwise installed.
+- [x] With the operator's own `git` and `tea` on the search path, those are the ones the binary invokes.
+- [x] The fast test tier runs and passes inside the build; a deliberately failing test fails the build.
+- [x] ADR 0018 is committed as part of this slice.
+- [x] The agent instructions carry a Gotcha about extending the source allowlist for new build-relevant files.
+
+## Implementation Notes
+
+### Three systems, not four — `x86_64-darwin` is gone
+
+The one dropped criterion. nixpkgs 26.11, which `nixos-unstable` now points at, has removed `x86_64-darwin` support outright.
+`legacyPackages.x86_64-darwin` *throws* on evaluation rather than merely failing to build, so enumerating it would break `nix flake show` and `nix flake check` for **every** system at once, not just that one.
+The flake therefore covers `x86_64-linux`, `aarch64-linux`, and `aarch64-darwin`, with the omission commented at the `systems` list.
+Intel macOS would need the 26.05 branch; nobody is asking for it.
+The rest of the criterion holds: nixpkgs is the only flake input.
+
+### `doCheck = true` was silently doing nothing
+
+`buildNpmPackage` wires config, build, and install hooks but supplies **no check hook**, so `doCheck` alone is inert.
+The first green build logged `no Makefile or custom checkPhase, doing nothing` and produced a package whose tests had never run — a passing build that verified nothing.
+The fix is an explicit `checkPhase`, plus `git` and `which` in `nativeCheckInputs` and a writable `HOME`.
+Recorded as a Gotcha, since the failure mode is a *green* build.
+
+The second half of that criterion was demonstrated unintentionally but genuinely: with the check phase live, a failing test failed the build with exit code 1 before anything was installed.
+
+### The build tree is named, and why that is a workaround
+
+`postUnpack` renames the build tree from the builder's generic `source` to `gitea-axi`.
+
+This is not cosmetic. `test/setup.test.ts` asserts that `setup hooks` updates its managed entry in place rather than appending a duplicate.
+The SDK recognises its own hook by testing whether the recorded command *string contains* `"gitea-axi"`, and the recorded command is the entrypoint's absolute path whenever PATH resolution does not match it — which it never does under vitest, since the entrypoint resolves to `src/main.js`, a file that does not exist.
+So the assertion holds only when the checkout's path happens to contain `gitea-axi`.
+
+That was verified rather than assumed: copying the repository to `/tmp/clean-probe-9d3/proj` and running the tier reproduces the failure outside Nix entirely.
+A first probe under the session scratchpad passed and was misleading — that path contains `-home-alexion-wrk-gitea-axi-…`, so it satisfied the substring by accident.
+
+The rename makes the build environment representative of a real installation (`node_modules/gitea-axi/…` under npm, `…-gitea-axi-<version>/…` under Nix) rather than an arrangement no operator ever has.
+It is a workaround for a defect, not a property worth keeping, and is commented as such.
+
+### Follow-up for task 0042 — the open verification item is answered, unfavourably
+
+The spec's open verification item asked whether the SDK prefers the bare binary name over the absolute entrypoint path.
+It does not.
+`resolvePortableHookCommand` returns the bare name only when a `PATH` entry realpath-matches the entrypoint, and the absolute path otherwise.
+
+Driving the Nix-built binary shows what actually lands in `~/.claude/settings.json`:
+
+```
+/nix/store/nmkzjny0hpzjvyxzdz189whk605di8b6-gitea-axi-0.1.0/lib/node_modules/gitea-axi/dist/main.js
+```
+
+That is content-addressed: it changes on every rebuild and is eventually garbage-collected, and a `SessionStart` hook that cannot execute simply does not run.
+So the dashboard stops appearing after an upgrade, silently and with nothing pointing at the cause — the exact failure the item hoped to rule out, now confirmed under the install method this slice adds.
+
+Two findings for 0042, both tracing to the same line:
+
+1. The stale store path above — user-facing breakage, and the more serious of the two.
+2. `setup hooks` appends a duplicate entry instead of updating in place whenever the entrypoint path lacks the marker.
+
+Both were left alone deliberately, at the maintainer's direction, to keep this slice about packaging; fixing hook resolution here would have pre-empted 0042's design work with a decision made in passing.
+When 0042 lands, the `postUnpack` rename should be removed with it.
+
+### ADR 0018
+
+Already committed in `0cbfe43` during the planning pass, ahead of this branch, so the criterion is satisfied by an earlier commit rather than by this one.
+No change was needed; `package.nix` implements it via `--suffix PATH`.
diff --git a/.claude/tasks/0042-verify-hook-path-resolution.md b/.claude/tasks/0042-verify-hook-path-resolution.md
index 59246ca..b89d8b6 100644
--- a/.claude/tasks/0042-verify-hook-path-resolution.md
+++ b/.claude/tasks/0042-verify-hook-path-resolution.md
@@ -23,3 +23,27 @@ Changing the `setup` command to prefer the bare name is explicitly **not** part
 - [ ] The finding is recorded where a future reader will meet it, so the question is not re-opened from scratch.
 - [ ] If the absolute path is recorded, the documentation states that hook setup must be re-run after an upgrade.
 - [ ] No change is made to how the `setup` command constructs the hook in this task.
+
+## Evidence gathered during task 0037
+
+Task 0037 built the flake, which made the SDK's behaviour directly observable.
+The answer is the unfavourable one: **the absolute path is recorded**, so the mitigation branch of this task applies, not the close-the-item branch.
+
+`resolvePortableHookCommand` in `axi-sdk-js` returns the bare binary name only when a `PATH` entry realpath-matches the entrypoint, and the absolute path in every other case.
+Driving the Nix-built binary writes this into `~/.claude/settings.json`:
+
+```
+/nix/store/nmkzjny0hpzjvyxzdz189whk605di8b6-gitea-axi-0.1.0/lib/node_modules/gitea-axi/dist/main.js
+```
+
+That path is content-addressed, so it changes on every rebuild and is eventually garbage-collected, and the session-start hook then silently stops running.
+
+A second defect surfaced from the same line.
+`isManagedHook` recognises its own hook by testing whether the recorded command *string contains* the marker `"gitea-axi"`, so when the entrypoint path lacks that substring, `setup hooks` appends a duplicate entry instead of updating in place — contradicting the idempotency its help text promises.
+This is reproducible outside Nix: copy the checkout to a path containing no `gitea-axi` segment and `test/setup.test.ts` fails.
+
+Consequences for this task:
+
+- Both defects trace to the same resolution line, so they should be weighed together.
+- The stale store path is user-facing breakage on the install method task 0037 added, which argues for not letting this drift far behind it.
+- `package.nix` carries a `postUnpack` rename of the build tree purely to work around the substring coupling. It is commented as a workaround and should be **deleted as part of this task**, once the hook no longer depends on the entrypoint path.
diff --git a/CLAUDE.md b/CLAUDE.md
index 884f5c9..ef6a463 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -14,7 +14,7 @@ Run `npm run build` before any live `bench:run` if you want `src/` changes refle
 
 Prefer this project's own CLI for pull requests — it is the tool being built, so opening its PRs with it is the dogfood path:
 `npm run build && node dist/main.js pr create --login alexion --base main --head <branch> --title <text> --body-file <path>`.
-It reuses the `tea` login store, which here holds exactly `alexion` and `csv-reviewer` — there is no `axi` profile.
+It reuses the `tea` login store, which here holds exactly `alexion` — there is no `axi` profile, and the `csv-reviewer` profile that used to exist is gone for good.
 `selectLogin` matches the `--login` value against those names exactly, so `--login alexion` works and an unknown name like `--login axi` fails with `VALIDATION_ERROR` ("Login profile "axi" not found").
 Fall back to `tea pr create --login alexion --base main --head <branch>` only for what gitea-axi cannot do yet; `tea pr` still lists PRs until `pr list` lands (task 0008).
 
@@ -34,3 +34,21 @@ The fixture tier is unaffected — it stubs the endpoint — so this bites only
 That ADR moved *command dispatch* to `gitea-js`; it explicitly kept `tea` for **credential discovery**, and its own Consequences section says so.
 Per [ADR 0001](.claude/adr/0001-diff-auth-via-tea-login-list.md) as amended, `src/context.ts` resolves auth by shelling out to `tea login list --output json` (discovery) and `tea login helper get --login <name>` (token, with in-place OAuth refresh).
 The only bypass is the test hook requiring `GITEA_AXI_API_URL` + `GITEA_AXI_TOKEN` + `GITEA_AXI_REPO` together; there is no user-facing path that avoids `tea`, and `TEA_NOT_INSTALLED` exists for its absence.
+
+Neither `node`/`npm` nor `tea` is on the `PATH` in a non-interactive shell on this machine, and there is no `~/.gitconfig`.
+This is a NixOS host with no global Node install, and the repository has no dev shell yet (task 0039 adds one).
+Until then, prefix commands with `nix shell nixpkgs#nodejs -c ...`, adding `nixpkgs#tea` for anything that resolves credentials — including `gitea-axi pr create`.
+`node_modules/` may be absent too, so `npm ci` first.
+Commits need an explicit identity: `git -c user.name=alexion -c user.email=contact@alexion.dev commit ...`, matching the existing history.
+
+The Nix derivation's source is an **explicit allowlist** in [`package.nix`](package.nix), not the whole repository and not a gitignore filter.
+A new build-relevant top-level file — a TypeScript configuration, a runner configuration the fast tier loads, a directory the build reads — must be added to that `lib.fileset.unions` list or `nix build` fails on a missing file.
+The failure is loud but disconnected from its cause: the error names the missing file, not the allowlist that omitted it.
+The flip side is the point of the design — touching an ADR, a spec, a task, a `bench/` file, or prose documentation must *not* change the derivation's output path.
+
+`buildNpmPackage` provides **no check hook**, so `doCheck = true` on its own is silently inert — the build logs `no Makefile or custom checkPhase, doing nothing` and ships a package whose tests never ran.
+Running the fast tier inside the derivation requires an explicit `checkPhase`; it also needs `git` and `which` in `nativeCheckInputs` and a writable `HOME`, since some of those tests shell out to `git`.
+
+nixpkgs 26.11 (the `nixos-unstable` the flake tracks) has **dropped `x86_64-darwin`**.
+`legacyPackages.x86_64-darwin` now *throws* rather than merely failing to build, so listing that system in the flake's `systems` breaks `nix flake show` and `nix flake check` for every system at once, not just that one.
+Intel macOS would need the 26.05 branch.
diff --git a/flake.lock b/flake.lock
new file mode 100644
index 0000000..301a9a7
--- /dev/null
+++ b/flake.lock
@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1784356753,
+        "narHash": "sha256-12KrbMiWLcf8m7pCvAtZh1ZrgF85ZXDXvfR/fWTKy84=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "61b7c44c4073f0b827768aff0049561b5110ea5a",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/flake.nix b/flake.nix
new file mode 100644
index 0000000..186f62e
--- /dev/null
+++ b/flake.nix
@@ -0,0 +1,31 @@
+{
+  description = "Agent-ergonomic CLI for Gitea issues and pull requests";
+
+  # Tracks unstable to match the maintainer's system. Consumers deduplicate by
+  # pointing this input at their own nixpkgs, so it governs standalone builds
+  # only — never the deployed artifact.
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      # x86_64-darwin is deliberately absent: nixpkgs 26.11 dropped it, and
+      # `legacyPackages.x86_64-darwin` now throws rather than merely failing to
+      # build — so listing it would break `nix flake show` and `nix flake check`
+      # for every system, not just that one. Intel macOS needs the 26.05 branch.
+      systems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "aarch64-darwin"
+      ];
+
+      forAllSystems =
+        f: nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});
+    in
+    {
+      packages = forAllSystems (pkgs: rec {
+        gitea-axi = pkgs.callPackage ./package.nix { };
+        default = gitea-axi;
+      });
+    };
+}
diff --git a/package.nix b/package.nix
new file mode 100644
index 0000000..fadf40d
--- /dev/null
+++ b/package.nix
@@ -0,0 +1,119 @@
+{
+  lib,
+  buildNpmPackage,
+  importNpmLock,
+  makeWrapper,
+  nodejs,
+  git,
+  tea,
+  which,
+}:
+
+let
+  # The manifest is the canonical version: the release flow bumps it, and
+  # reading it here means a store path and a released version cannot disagree.
+  manifest = lib.importJSON ./package.json;
+
+  # An explicit allowlist of what the build and its tests actually read. The
+  # repository's highest-churn directories — .claude, bench, prose docs — are
+  # all build-irrelevant, so a whole-repository source would let writing an ADR
+  # invalidate the derivation and force a rebuild with a full test run.
+  #
+  # Adding a build-relevant top-level file means adding it here too; the build
+  # otherwise fails on a missing file.
+  src = lib.fileset.toSource {
+    root = ./.;
+    fileset = lib.fileset.unions [
+      ./src
+      # The fast tier only. `test/e2e` needs a live Gitea host and is excluded
+      # from the runner config, so admitting it would let end-to-end churn
+      # invalidate the derivation — the very cost this allowlist exists to
+      # avoid. `test/packaging` stays: task 0038 drives it against the
+      # installed binary.
+      (lib.fileset.difference ./test ./test/e2e)
+      ./skills
+      ./package.json
+      ./package-lock.json
+      ./tsconfig.json
+      ./tsconfig.build.json
+      ./vitest.config.ts
+    ];
+  };
+in
+buildNpmPackage {
+  pname = "gitea-axi";
+  inherit (manifest) version;
+  inherit src nodejs;
+
+  # Each dependency's fetch is derived from the integrity fields already in the
+  # lockfile, so a lockfile change needs no edit here. A single fixed-output
+  # hash would break on every dependency bump and be repaired by copying a hash
+  # out of an error message — a permanent recurring tax.
+  npmDeps = importNpmLock { npmRoot = src; };
+  inherit (importNpmLock) npmConfigHook;
+
+  nativeBuildInputs = [ makeWrapper ];
+
+  # The builder would otherwise unpack to a generic `source` directory, which no
+  # real installation resembles: under npm the tree lives at
+  # `node_modules/gitea-axi`, under Nix at `…-gitea-axi-<version>/…`. The fast
+  # tier's `setup hooks` test is sensitive to the difference, because the SDK
+  # records the entrypoint's absolute path and recognises its own managed hook by
+  # finding "gitea-axi" within it. Naming the tree makes the build representative
+  # rather than an environment no operator ever has.
+  #
+  # This coupling is a defect, not a property worth preserving — see task 0042,
+  # which removes the hook's dependence on the entrypoint path entirely. Once it
+  # lands this rename should go with it.
+  postUnpack = ''
+    mv "$sourceRoot" gitea-axi
+    export sourceRoot=gitea-axi
+  '';
+
+  # The fast tier only. The live end-to-end and benchmark smoke tiers need a
+  # live Gitea host. Two of these test files invoke `git` directly and one
+  # resolves it with `which`; `tea` is already stubbed within this tier.
+  doCheck = true;
+  nativeCheckInputs = [
+    git
+    which
+  ];
+
+  # `buildNpmPackage` wires config, build and install hooks but no check hook, so
+  # `doCheck` alone is inert and the phase has to be spelled out. `git init` and
+  # `git commit` in the fast tier also need a writable HOME, which the sandbox
+  # otherwise points at a non-existent directory.
+  checkPhase = ''
+    runHook preCheck
+
+    export HOME=$(mktemp -d)
+    npm run test
+
+    runHook postCheck
+  '';
+
+  # ADR 0018: append, never prepend. The operator's own `tea` owns the
+  # credential store it refreshes in place, so the closure's copy is a
+  # fresh-machine fallback rather than an override.
+  postInstall = ''
+    wrapProgram $out/bin/gitea-axi \
+      --suffix PATH : ${lib.makeBinPath [ git tea ]}
+  '';
+
+  meta = {
+    inherit (manifest) description homepage;
+
+    # Looked up by SPDX identifier rather than hardcoded, for the same reason
+    # the version is read from the manifest: one canonical source, no second
+    # place to update on a relicence.
+    license = lib.licensesSpdx.${manifest.license};
+
+    mainProgram = "gitea-axi";
+
+    # Broader than the flake's `systems` list, deliberately. This describes what
+    # the package supports — everything, since it contains no compiled code —
+    # whereas that list encodes which systems the pinned nixpkgs can still
+    # evaluate. Consumed against 26.05, x86_64-darwin builds fine from here.
+    platforms = lib.platforms.linux ++ lib.platforms.darwin;
+  };
+}