fix(pairing): persist shortcut commands and avoid Feishu side effects

- AgentLoop._state_command now persists user message and assistant
  response for shortcut commands (e.g. /pairing) so WebUI history
  hydration after _turn_end no longer shows an empty chat.  /new is
  excluded because it intentionally clears the session.

- Feishu _on_message sends pairing codes for unauthorized DMs before
  any media side effects (reactions, downloads, transcription).
  Group chat unauthorized senders are still silently ignored early.

- Update test_feishu_reply to assert the new DM pairing behavior.

nanobot/channels/feishu.py

@@ -1712,6 +1712,18 @@ class FeishuChannel(BaseChannel):
             while len(self._processed_message_ids) > 1000:
                 self._processed_message_ids.popitem(last=False)
 
+            # Early permission check — avoid side effects for unauthorized users.
+            # Group chats are silently ignored; DMs get a pairing code.
+            if not self.is_allowed(sender_id):
+                if chat_type == "p2p":
+                    await self._handle_message(
+                        sender_id=sender_id,
+                        chat_id=sender_id,
+                        content="",
+                        is_dm=True,
+                    )
+                return
+
             # Add reaction (non-blocking — tracked background task)
             task = asyncio.create_task(
                 self._add_reaction(message_id, self.config.react_emoji)