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/agent/test_loop_save_turn.py

@@ -602,17 +602,17 @@ async def test_process_message_uses_explicit_session_metadata_for_goal_context(
     chat_session = loop.sessions.get_or_create("websocket:chat-with-goal")
     chat_session.metadata[GOAL_STATE_KEY] = {
         "status": "active",
-        "objective": "This chat goal must not leak into heartbeat.",
+        "objective": "This chat goal must not leak into system.",
     }
     loop.sessions.save(chat_session)
-    system_session = loop.sessions.get_or_create("heartbeat")
+    system_session = loop.sessions.get_or_create("system")
     system_session.metadata = {}
     loop.sessions.save(system_session)
 
     loop.context.build_messages = MagicMock(  # type: ignore[method-assign]
         return_value=[
             {"role": "system", "content": "system"},
-            {"role": "user", "content": "runtime + heartbeat"},
+            {"role": "user", "content": "runtime + system"},
         ]
     )
     loop._run_agent_loop = AsyncMock(return_value=(  # type: ignore[method-assign]
@@ -620,7 +620,7 @@ async def test_process_message_uses_explicit_session_metadata_for_goal_context(
         [],
         [
             {"role": "system", "content": "system"},
-            {"role": "user", "content": "runtime + heartbeat"},
+            {"role": "user", "content": "runtime + system"},
             {"role": "assistant", "content": "ok"},
         ],
         "stop",
@@ -630,11 +630,11 @@ async def test_process_message_uses_explicit_session_metadata_for_goal_context(
     result = await loop._process_message(
         InboundMessage(
             channel="websocket",
-            sender_id="heartbeat",
+            sender_id="system",
             chat_id="chat-with-goal",
-            content="heartbeat work",
+            content="system work",
         ),
-        session_key="heartbeat",
+        session_key="system",
     )
 
     assert result is not None