feat: add heartbeat service for periodic task checking

Adds a heartbeat timer that reads /spiffs/config/HEARTBEAT.md every 30
minutes and sends a prompt to the agent if actionable tasks are found.
Skips empty lines, headers, and completed checkboxes. Includes a
heartbeat_trigger CLI command for manual testing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main/CMakeLists.txt

@@ -28,6 +28,7 @@ idf_component_register(
         "tools/tool_files.c"
         "tools/tool_cron.c"
         "cron/cron_service.c"
+        "heartbeat/heartbeat.c"
     INCLUDE_DIRS
         "."
     EMBED_FILES