suppress unnecessary heartbeat notifications

Appends a strict instruction to background task prompts (cron and heartbeat) 
directing the agent to return a `<SILENT_OK>` token if there is nothing 
material to report. Adds conditional logic to intercept this token and 
suppress the outbound message to the user, preventing notification spam 
from autonomous background checks.

nanobot/heartbeat/service.py

@@ -153,9 +153,15 @@ class HeartbeatService:
                 logger.info("Heartbeat: OK (nothing to report)")
                 return
 
+            taskmessage = tasks + "\n\n**IMPORTANT NOTICE:** If there is nothing material to report, reply only with <SILENT_OK>."
+
             logger.info("Heartbeat: tasks found, executing...")
             if self.on_execute:
-                response = await self.on_execute(tasks)
+                response = await self.on_execute(taskmessage)
+
+                if response and "<SILENT_OK>" in response:
+                    logger.info("Heartbeat: OK (silenced by agent)")
+                    return
                 if response and self.on_notify:
                     logger.info("Heartbeat: completed, delivering response")
                     await self.on_notify(response)