#762 Durable cron jobs don't recover after worker restart

All durable cron jobs stopped running after internal worker restart at ~17:50 UTC. No recovery mechanism exists to respawn dead cron chains. Impact: stale_worker_cleanup, stuck_task_detector, workflow_state_cleanup, zombie_container_reaper, activity_timeout_scanner all stopped. Root cause: spawn_next step commits checkpoint but task spawn may be lost during worker death.