Rebase onto master (#443 keyed store) and port #438 hardening: per-issue
fcntl.flock around bind_session_lock, assess_lock_freshness with dead-PID
detection, verify_lock_for_mutation before create_pr, and live lock
inventory for claim visibility.
Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>