Scan all session files on startup and recover any with stale turn state
(pending_user_turn or runtime_checkpoint metadata) left from a previous
crash or restart. Previously this only happened lazily when a new message
arrived for the affected session.
- Add SessionManager.list_session_keys() to enumerate session keys from disk
- Add AgentLoop.recover_stale_sessions() to scan and recover all sessions
- Call recover_stale_sessions() during gateway startup in _run_gateway
