feat(nix): package gitea-axi as a flake with a PATH-suffixing wrapper (task 0037)

Add a flake at the repository root exposing gitea-axi as a package, with the
derivation in its own callable expression so it stays buildable outside a flake
context and usable in an overlay unchanged.

Dependencies are fetched from the lockfile's integrity fields via importNpmLock
rather than a committed fixed-output hash, and the version is read from the
package manifest, so neither a dependency bump nor a release edits any Nix
expression. The source is an explicit allowlist, keeping ADR, spec, task and
bench churn out of the derivation's inputs.

The installed binary is wrapped with --suffix PATH per ADR 0018: the operator's
own git and tea win, and the closure's are a fresh-machine fallback.

Two things differ from the plan. Systems coverage is three targets, not four:
nixpkgs 26.11 dropped x86_64-darwin and now throws on evaluating it, which
would break nix flake show and nix flake check for every system at once. And
buildNpmPackage supplies no check hook, so doCheck alone was silently inert and
produced a green build whose tests never ran; running the fast tier needs an
explicit checkPhase.

.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`.

.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.