suppress unnecessary cron 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/cli/commands.py

@@ -452,6 +452,7 @@ def gateway(
             "[Scheduled Task] Timer finished.\n\n"
             f"Task '{job.name}' has been triggered.\n"
             f"Scheduled instruction: {job.payload.message}"
+            "**IMPORTANT NOTICE:** If there is nothing material to report, reply only with <SILENT_OK>."
         )
 
         # Prevent the agent from scheduling new cron jobs during execution
@@ -474,7 +475,7 @@ def gateway(
         if isinstance(message_tool, MessageTool) and message_tool._sent_in_turn:
             return response
 
-        if job.payload.deliver and job.payload.to and response:
+        if job.payload.deliver and job.payload.to and response and "<SILENT_OK>" not in response:
             from nanobot.bus.events import OutboundMessage
             await bus.publish_outbound(OutboundMessage(
                 channel=job.payload.channel or "cli",