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.

tests/cli/test_commands.py

@@ -1589,16 +1589,6 @@ def test_gateway_health_endpoint_binds_and_serves_expected_responses(
         def register_system_job(self, _job) -> None:
             return None
 
-    class _FakeHeartbeatService:
-        def __init__(self, **_kwargs) -> None:
-            return None
-
-        async def start(self) -> None:
-            return None
-
-        def stop(self) -> None:
-            return None
-
     class _FakeServer:
         async def __aenter__(self):
             return self
@@ -1645,7 +1635,6 @@ def test_gateway_health_endpoint_binds_and_serves_expected_responses(
     monkeypatch.setattr("nanobot.cli.commands.AgentLoop", _FakeAgentLoop)
     monkeypatch.setattr("nanobot.channels.manager.ChannelManager", _FakeChannelManager)
     monkeypatch.setattr("nanobot.cron.service.CronService", _FakeCronService)
-    monkeypatch.setattr("nanobot.heartbeat.service.HeartbeatService", _FakeHeartbeatService)
     monkeypatch.setattr("asyncio.start_server", _fake_start_server)
 
     result = runner.invoke(app, ["gateway", "--config", str(config_file)])