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.

core_agent_lines.sh

@@ -46,17 +46,15 @@ core_agent=$(count_top_level_py_lines "nanobot/agent")
 core_bus=$(count_top_level_py_lines "nanobot/bus")
 core_config=$(count_top_level_py_lines "nanobot/config")
 core_cron=$(count_top_level_py_lines "nanobot/cron")
-core_heartbeat=$(count_top_level_py_lines "nanobot/heartbeat")
 core_session=$(count_top_level_py_lines "nanobot/session")
 
 print_row "agent/" "$core_agent"
 print_row "bus/" "$core_bus"
 print_row "config/" "$core_config"
 print_row "cron/" "$core_cron"
-print_row "heartbeat/" "$core_heartbeat"
 print_row "session/" "$core_session"
 
-core_total=$((core_agent + core_bus + core_config + core_cron + core_heartbeat + core_session))
+core_total=$((core_agent + core_bus + core_config + core_cron + core_session))
 
 echo ""
 echo "Separate buckets"