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/utils/helpers.py

@@ -626,3 +626,14 @@ def sync_workspace_templates(workspace: Path, silent: bool = False) -> list[str]
         logger.exception("Failed to initialize git store for {}", workspace)
 
     return added
+
+
+def load_bundled_template(template_name: str) -> str | None:
+    """Read a bundled template file from the nanobot package."""
+    from importlib.resources import files as pkg_files
+
+    with suppress(Exception):
+        tpl = pkg_files("nanobot") / "templates" / template_name
+        if tpl.is_file():
+            return tpl.read_text(encoding="utf-8")
+    return None