fix: prevent cron job execution from scheduling new jobs

When a cron job fires, the agent processes the scheduled message and
has access to the cron tool. If the original message resembles a
scheduling instruction (e.g. "remind me in 10 seconds"), the agent
would call cron.add again, creating an infinite feedback loop.

Add a cron-context flag to CronTool that blocks add operations during
cron job execution. The flag is set before process_direct() and cleared
in a finally block to ensure cleanup even on errors.

Fixes #1441

nanobot/cli/commands.py

@@ -296,6 +296,7 @@ def gateway(
     # Set cron callback (needs agent)
     async def on_cron_job(job: CronJob) -> str | None:
         """Execute a cron job through the agent."""
+        from nanobot.agent.tools.cron import CronTool
         from nanobot.agent.tools.message import MessageTool
         reminder_note = (
             "[Scheduled Task] Timer finished.\n\n"
@@ -303,12 +304,20 @@ def gateway(
             f"Scheduled instruction: {job.payload.message}"
         )
 
-        response = await agent.process_direct(
-            reminder_note,
-            session_key=f"cron:{job.id}",
-            channel=job.payload.channel or "cli",
-            chat_id=job.payload.to or "direct",
-        )
+        # Prevent the agent from scheduling new cron jobs during execution
+        cron_tool = agent.tools.get("cron")
+        if isinstance(cron_tool, CronTool):
+            cron_tool.set_cron_context(True)
+        try:
+            response = await agent.process_direct(
+                reminder_note,
+                session_key=f"cron:{job.id}",
+                channel=job.payload.channel or "cli",
+                chat_id=job.payload.to or "direct",
+            )
+        finally:
+            if isinstance(cron_tool, CronTool):
+                cron_tool.set_cron_context(False)
 
         message_tool = agent.tools.get("message")
         if isinstance(message_tool, MessageTool) and message_tool._sent_in_turn: