fix(setup): record the session-start hook as a search-path name (task 0043)

The SDK records a bare, upgrade-stable binary name only when a PATH entry
realpath-matches the execPath it is handed. From the module-relative
entrypoint that can only succeed under npm, which symlinks its bin entry
straight at it; a wrapper-based install never can, because a script that
invokes a file does not resolve to that file. So every wrapper install --
Nix, a shim, a generated .cmd -- recorded an absolute path that moves on
upgrade, and a session-start hook that cannot execute fails silently.

`setup hooks` now resolves gitea-axi on PATH itself and hands that
location to the SDK, so the bare name is recorded. A candidate qualifies
only if it resolves to the running entrypoint -- by realpath for a
symlink, or by naming it in its text for a wrapper, following the chain,
since a Nix install is two hops. A same-named binary that is some other
program does not qualify, and the absolute entrypoint path stands as the
fallback exactly as before.

The related defect on the same line goes too: the SDK recognises its hook
by finding the marker inside the recorded command, so an entrypoint path
without "gitea-axi" in it made a re-run append a duplicate rather than
update in place. `setup hooks` now prunes duplicates by matching the
exact command it records, which is independent of that command's shape
and cannot mistake another tool's hook for its own.

With the coupling gone, package.nix no longer renames its build tree; the
fast tier runs from /build/source and its idempotency test passes there.
The help text's instruction to re-run hooks after an upgrade is deleted,
having become false.

Verified against the built Nix binary and a globally npm-installed pack:
both record the bare name, both fall back to the absolute path when the
name is absent, an impostor on PATH is refused, and re-runs leave one
entry. Decision recorded as ADR 0019; ADR 0009's addendum is amended.

package.nix

@@ -60,25 +60,6 @@ buildNpmPackage {
 
   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. Task 0042
-  # verified the resolution behaviour and documented the mitigation, but left
-  # the hook's dependence on the entrypoint path in place: removing it needs its
-  # own ADR, since a stable search-path name is the right answer for every
-  # installation method and not a Nix special case. This rename goes away with
-  # that task, not before.
-  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.