refactor(agent): pin session activity heartbeat cadence + harden best-effort write

Heartbeat write discipline for the durable SessionDB activity projection:

- Pin the cadence in a named constant
  (SESSION_ACTIVITY_HEARTBEAT_MIN_INTERVAL_SECONDS = 60s, contract >= 30s,
  deliberately config-independent so no compression.*/agent.* setting can
  turn the heartbeat into a high-frequency writer on the contended
  SessionDB write path).
- The write already rides the standard _execute_write patience path via
  SessionDB.touch_session_activity — verified, now documented in the
  docstring.
- Best-effort hardening: a failed heartbeat write never raises into the
  agent loop; the bare 'pass' becomes an explicit debug log with traceback.
- Tests: direct proof that a heartbeat DB failure doesn't propagate,
  the cadence constant is pinned >= 30s, and the rate limiter keys off
  the shared constant (boundary tested on both sides of the window).

agent/session_activity.py

@@ -18,6 +18,16 @@ from typing import Any, Mapping, Optional
 
 ACTIVITY_DESCRIPTION_MAX = 120
 
+# Durable SessionDB activity heartbeat cadence (seconds between writes per
+# session). Contract: MUST stay >= 30s — the SessionDB write path is
+# contended (deadline/patience retry, compression-lock patience), and the
+# heartbeat is an observation-only projection that never justifies extra
+# write pressure. This cadence is deliberately a code constant, independent
+# of any compression.* or agent.* config, so no configuration can turn the
+# heartbeat into a high-frequency writer. Matches the kanban auto-heartbeat
+# cadence. force_persist (terminal stamps) is the only bypass.
+SESSION_ACTIVITY_HEARTBEAT_MIN_INTERVAL_SECONDS = 60.0
+
 
 class ActivityProvenance(str, Enum):
     """Where a durable/in-memory activity stamp came from."""