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/cli/onboard.py

@@ -1155,7 +1155,7 @@ _SETTINGS_SECTIONS: dict[str, tuple[str, str, set[str] | None]] = {
     "Agent Settings": ("Agent Defaults", "Configure default model, temperature, and behavior", None),
     "Channel Common": ("Channel Common", "Configure cross-channel behavior: progress, tool hints, retries", None),
     "API Server": ("API Server", "Configure OpenAI-compatible API endpoint", None),
-    "Gateway": ("Gateway Settings", "Configure server host, port, and heartbeat", None),
+    "Gateway": ("Gateway Settings", "Configure server host, port", None),
     "Tools": ("Tools Settings", "Configure web search, shell exec, and other tools", {"mcp_servers"}),
 }