refactor(heartbeat): migrate heartbeat service to cron-based auto-registration

Remove standalone nanobot/heartbeat/ service and replace it with an
auto-registered system cron job on gateway startup. Key behaviors preserved:

- HeartbeatConfig (enabled, interval_s, keep_recent_messages) remains in
  GatewayConfig for backward compatibility.
- On startup, if enabled, a system cron job "heartbeat" is registered with
  schedule derived from interval_s.
- HEARTBEAT.md is checked on each tick; empty/template-identical files skip
  to avoid wasting LLM calls.
- Post-run evaluate_response and session history truncation
  (keep_recent_messages) are retained.
- Delivery target selection, deliverable filtering, and preamble guidance
  are preserved.

Files removed:
- nanobot/heartbeat/__init__.py
- nanobot/heartbeat/service.py
- tests/heartbeat/*
- tests/agent/test_heartbeat_service.py

Templates and docs updated to reflect cron-based usage.

nanobot/config/schema.py

@@ -236,7 +236,7 @@ class ProvidersConfig(Base):
 
 
 class HeartbeatConfig(Base):
-    """Heartbeat service configuration."""
+    """Heartbeat service configuration (now backed by cron)."""
 
     enabled: bool = True
     interval_s: int = 30 * 60  # 30 minutes