fix(kanban): stale reclaim must not tick failure counter (#28680)

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.

agent/prompt_builder.py

@@ -206,7 +206,12 @@ KANBAN_GUIDANCE = (
     "files outside it unless the task explicitly asks.\n"
     "3. **Heartbeat on long operations.** Call `kanban_heartbeat(note=...)` "
     "every few minutes during long subprocesses (training, encoding, crawling). "
-    "Skip heartbeats for short tasks.\n"
+    "Skip heartbeats for short tasks. **If your task may run longer than 1 hour, "
+    "you MUST call `kanban_heartbeat` at least once an hour** — the dispatcher "
+    "reclaims tasks running past `kanban.dispatch_stale_timeout_seconds` "
+    "(default 4 hours) when no heartbeat has arrived in the last hour. A "
+    "reclaim re-queues the task as `ready` without penalty (no failure counter "
+    "tick), but you lose your current run's progress.\n"
     "4. **Block on genuine ambiguity.** If you need a human decision you cannot "
     "infer (missing credentials, UX choice, paywalled source, peer output you "
     "need first), call `kanban_block(reason=\"...\")` and stop. Don't guess. "