fix(agent): only suppress final reply when message tool sends to same target

A refactoring in commit 132807a introduced a regression where the final
response was silently discarded whenever the message tool was used,
regardless of the target. This restored the original logic from PR #832
that only suppresses the final reply when the message tool sends to the
same (channel, chat_id) as the original message.

Changes:
- message.py: Replace _sent_in_turn: bool with _turn_sends: list[tuple]
  to track actual send targets, add get_turn_sends() method
- loop.py: Check if (msg.channel, msg.chat_id) is in sent_targets before
  suppressing final reply. Also move the "Response to" log after the
  suppress check to avoid misleading logs.
- Add unit tests for the suppress logic

This ensures:
- Email sent via message tool → Feishu still gets confirmation
- Message tool sends to same Feishu chat → No duplicate (suppressed)

nanobot/agent/loop.py

@@ -407,16 +407,25 @@ class AgentLoop:
         if final_content is None:
             final_content = "I've completed processing but have no response to give."
 
-        preview = final_content[:120] + "..." if len(final_content) > 120 else final_content
-        logger.info("Response to {}:{}: {}", msg.channel, msg.sender_id, preview)
-
         self._save_turn(session, all_msgs, 1 + len(history))
         self.sessions.save(session)
 
+        suppress_final_reply = False
         if message_tool := self.tools.get("message"):
-            if isinstance(message_tool, MessageTool) and message_tool._sent_in_turn:
-                return None
+            if isinstance(message_tool, MessageTool):
+                sent_targets = set(message_tool.get_turn_sends())
+                suppress_final_reply = (msg.channel, msg.chat_id) in sent_targets
 
+        if suppress_final_reply:
+            logger.info(
+                "Skipping final auto-reply because message tool already sent to {}:{} in this turn",
+                msg.channel,
+                msg.chat_id,
+            )
+            return None
+
+        preview = final_content[:120] + "..." if len(final_content) > 120 else final_content
+        logger.info("Response to {}:{}: {}", msg.channel, msg.sender_id, preview)
         return OutboundMessage(
             channel=msg.channel, chat_id=msg.chat_id, content=final_content,
             metadata=msg.metadata or {},