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/agent/context.py

@@ -3,8 +3,6 @@
 import base64
 import mimetypes
 import platform
-from contextlib import suppress
-from importlib.resources import files as pkg_files
 from pathlib import Path
 from typing import Any, Mapping, Sequence
 
@@ -12,12 +10,13 @@ from nanobot.agent.memory import MemoryStore
 from nanobot.agent.skills import SkillsLoader
 from nanobot.agent.tools import mcp as mcp_tools
 from nanobot.agent.tools.registry import ToolRegistry
-from nanobot.bus.events import InboundMessage
 from nanobot.apps.cli import utils as cli_app_utils
+from nanobot.bus.events import InboundMessage
 from nanobot.session.goal_state import goal_state_runtime_lines
 from nanobot.utils.helpers import (
     current_time_str,
     detect_image_mime,
+    load_bundled_template,
     truncate_text,
 )
 from nanobot.utils.prompt_templates import render_template
@@ -168,10 +167,9 @@ class ContextBuilder:
     @staticmethod
     def _is_template_content(content: str, template_path: str) -> bool:
         """Check if *content* is identical to the bundled template (user hasn't customized it)."""
-        with suppress(Exception):
-            tpl = pkg_files("nanobot") / "templates" / template_path
-            if tpl.is_file():
-                return content.strip() == tpl.read_text(encoding="utf-8").strip()
+        tpl = load_bundled_template(template_path)
+        if tpl is not None:
+            return content.strip() == tpl.strip()
         return False
 
     def build_messages(