Follow-up to #28452. detect_stale_running() was calling
_record_task_failure() on every reclaim, which ticked the
consecutive_failures counter. With the default failure_limit=2,
two legitimately long-running tasks (>4 h without explicit
heartbeat) would auto-block via the spawn-failure circuit
breaker — even though no worker actually failed.
Stale reclaim is dispatcher-side absence-of-heartbeat detection,
not a worker fault. Removed the _record_task_failure() call;
the 'stale' event in task_events is still the audit surface,
but the failure counter is now reserved for spawn_failed /
timed_out / crashed (real failures).
Also documents the heartbeat requirement:
- KANBAN_GUIDANCE in agent/prompt_builder.py now states the
rule ('call kanban_heartbeat at least once an hour for tasks
running longer than 1 hour') so workers learn the contract.
- kanban.md adds the stale event row to the events table and
flags the heartbeat requirement in the worker lifecycle list.
New regression test: test_detect_stale_does_not_tick_failure_counter
locks in the new behaviour.
