fix: address Copilot review round 2 (#2339)

- The holder-record removal is best-effort and can no longer throw past
  release(); ERELEASED on release is the library having already dropped a
  taken-over lock, so it is ignored rather than described as a lock that
  "goes stale on its own".
- The signal test's stand-in now spawns a descendant of its own, so the
  whole-process-group guarantee is what the test asserts: a regression to
  child.kill() or a non-detached spawn leaves the descendant alive and fails.
- The three docs no longer claim a dead holder is always recovered in 30s:
  a stale lock directory that cannot be removed runs the wait to its
  45-minute cap and names the path.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PsmUAZ6aLUAFqenjJKSSRp
Signed-off-by: cliffhall <cliff@futurescale.com>

.claude/skills/pre-push-gate/SKILL.md

@@ -173,8 +173,12 @@ releases (it re-checks every 2s and prints `still waiting` once a minute). The
 holder's pid and worktree are in the line, so you can decide whether to wait
 or to stop that gate. A holder that was **killed** — a closed terminal, an
 OOM'd session — stops refreshing its lock and is taken over after 30s; nothing
-needs cleaning up by hand. The wait gives up after 45 minutes, naming the
-holder, which only ever happens against a live gate that has hung.
+needs cleaning up by hand. The one exception is a dead holder's lock directory
+that cannot be removed (a stray file inside it, or permissions): the takeover
+fails, the waiter keeps waiting, and the wait runs to its 45-minute cap naming
+the path — remove that directory by hand. So the give-up happens against a
+live gate that has hung, or a stale lock that would not go away; never on its
+own.
 
 `INSPECTOR_SKIP_GATE_LEASE=1 npm run local:gate` runs without the lease. It is
 for a measurement that needs contention; it does not get a result sooner,