refactor: replace <SILENT_OK> with structured post-run evaluation

- Add nanobot/utils/evaluator.py: lightweight LLM tool-call to decide notify/silent after background task execution
- Remove magic token injection from heartbeat and cron prompts
- Clean session history (no more <SILENT_OK> pollution)
- Add tests for evaluator and updated heartbeat three-phase flow

nanobot/cli/commands.py

@@ -448,14 +448,14 @@ def gateway(
         """Execute a cron job through the agent."""
         from nanobot.agent.tools.cron import CronTool
         from nanobot.agent.tools.message import MessageTool
+        from nanobot.utils.evaluator import evaluate_response
+
         reminder_note = (
             "[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
         cron_tool = agent.tools.get("cron")
         cron_token = None
         if isinstance(cron_tool, CronTool):
@@ -475,13 +475,17 @@ 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 and "<SILENT_OK>" not in response:
-            from nanobot.bus.events import OutboundMessage
-            await bus.publish_outbound(OutboundMessage(
-                channel=job.payload.channel or "cli",
-                chat_id=job.payload.to,
-                content=response
-            ))
+        if job.payload.deliver and job.payload.to and response:
+            should_notify = await evaluate_response(
+                response, job.payload.message, provider, agent.model,
+            )
+            if should_notify:
+                from nanobot.bus.events import OutboundMessage
+                await bus.publish_outbound(OutboundMessage(
+                    channel=job.payload.channel or "cli",
+                    chat_id=job.payload.to,
+                    content=response,
+                ))
         return response
     cron.on_job = on_cron_job