#143 CRITICAL: Risk of Double Execution due to Manual Heartbeating

Long-running tasks (> claim_timeout) are killed by other workers via claim_task logic if they do not manually call ctx.heartbeat(). This relies entirely on user correctness. If a user forgets to heartbeat, valid long-running tasks will be killed and potentially retried, leading to wasted compute or double execution (if non-idempotent). System should support automatic background heartbeating.