fix: add deferred trim to system message handler

System messages (including subagents) can trigger memory_consolidate,
which sets _trim_checkpoint. The system handler must also check and
apply deferred trims to prevent unbounded session growth.

Addresses review feedback from PR #17.

nanobot/agent/loop.py

@@ -803,6 +803,16 @@ class AgentLoop:
             session.add_raw_message(chain_msg)
         self.sessions.save(session)
 
+        # Deferred trim: same logic as _process_message
+        # System messages (including subagents) can trigger consolidation
+        checkpoint = getattr(session, '_trim_checkpoint', None)
+        if checkpoint is not None:
+            old_size = len(session.messages)
+            session.messages = session.messages[checkpoint:]
+            session._trim_checkpoint = None
+            self.sessions.save(session)
+            logger.info(f"Deferred trim applied: {old_size} -> {len(session.messages)} messages (checkpoint={checkpoint})")
+
         # Return original content (not signed) for outbound, but with suppressed metadata
         return OutboundMessage(
             channel=origin_channel,