chore: gitignore go.work for /tmp worktree resolution

go.work resolves the fn-registry replace to an absolute path when the worktree
lives outside the parent tree; it is local-only and must not be committed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -14,3 +14,7 @@ worker.id
 /chat
 *.exe
 registry.db
+
+# local worktree resolution (do not commit)
+go.work
+go.work.sum