Turn the memorized #119 near-miss LAW into a mechanical guard. ### Acceptance criteria - [ ] PreToolUse hook on `gh pr merge` blocks unless the PR `head.sha` equals the last pushed SHA AND check-runs exist for that exact SHA. - [ ] Emits a clear reason string on block. - [ ] Foreground-test the poll command once before arming (CLAUDE.md poll-loop gotcha).
Turn the memorized #119 near-miss LAW into a mechanical guard.
Acceptance criteria
gh pr mergeblocks unless the PRhead.shaequals the last pushed SHA AND check-runs exist for that exact SHA.