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/heartbeat/heartbeat.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "esp_err.h"
+
+/**
+ * Initialize the heartbeat service (logs ready state).
+ */
+esp_err_t heartbeat_init(void);
+
+/**
+ * Start the heartbeat timer. Checks HEARTBEAT.md periodically
+ * and sends a prompt to the agent if actionable tasks are found.
+ */
+esp_err_t heartbeat_start(void);
+
+/**
+ * Stop and delete the heartbeat timer.
+ */
+void heartbeat_stop(void);
+
+/**
+ * Manually trigger a heartbeat check (for CLI testing).
+ * Returns true if the agent was prompted, false if no tasks found.
+ */
+bool heartbeat_trigger(void);