docs: record that the session-start hook stores an absolute path (task 0042)

Resolve the nix-flake-packaging spec's open verification item by observation
rather than inference, and act on the unfavourable answer.

`resolvePortableHookCommand` in axi-sdk-js returns the bare binary name only
when a PATH entry realpath-matches the entrypoint. npm symlinks its bin entry
straight at dist/main.js and satisfies that; Nix installs a generated wrapper
script whose realpath is the wrapper, so the absolute store path is recorded
instead. Verified by probing the SDK with both install shapes and by driving
the flake-built binary against a temporary HOME.

The path is content-addressed, so it moves on every rebuild and is eventually
collected, and a session-start hook that cannot execute fails silently. The
mitigation is documentation, per the decision recorded when the item was
opened: the setup help text now says to re-run `setup hooks` after an upgrade.

How the setup command constructs the hook is deliberately unchanged. Preferring
the bare name is the right answer for every wrapper-based install, not a Nix
special case, so it belongs in a successor task with its own ADR alongside the
related `isManagedHook` substring defect. package.nix's postUnpack rename
therefore stays; its comment no longer promises this task will remove it.

src/commands/setup.ts

@@ -18,6 +18,14 @@ Install gitea-axi's ambient context for agent sessions.
 Both are idempotent: re-running updates the managed files in place rather than
 failing. There is no postinstall script — installation is always explicit.
 
+Re-run "setup hooks" after upgrading gitea-axi. The hook records an absolute
+path to the entrypoint, which moves when the install location changes, and a
+session-start hook that cannot be executed fails silently rather than warning.
+This matters most for immutable installs such as Nix, where every rebuild lands
+the entrypoint at a fresh path and the old one is eventually collected. When the
+path moves, the re-run may leave the stale entry behind instead of replacing it;
+remove it by hand if a duplicate appears.
+
 flags:
   --help           Show this help
 `;