feat(pairing): chat-native DM sender approval

Replace the file-editing onboarding workflow with a chat-native pairing flow:

- New pairing store (nanobot/pairing/store.py) persists approved senders
  and pending codes in ~/.nanobot/pairing.json.
- DM messages from unknown senders receive a short pairing code instead of
  silent denial. Group chats remain silently ignored.
- Existing allowFrom semantics are fully preserved; approved pairing users
  are merged at runtime so no config migration is needed.
- nanobot pairing list/approve/deny/revoke CLI commands for bootstrap and
  emergency management.
- /pairing slash commands intercepted in-channel so owners can approve
  senders without leaving the chat.
- is_dm flag added to BaseChannel._handle_message; Telegram, Discord and
  WebSocket updated to pass it.

Closes #3768

nanobot/channels/telegram.py

@@ -1011,6 +1011,7 @@ class TelegramChannel(BaseChannel):
             content=content,
             metadata=self._build_message_metadata(message, user),
             session_key=self._derive_topic_session_key(message),
+            is_dm=message.chat.type == "private",
         )
 
     async def _on_message(self, update: Update, context: ContextTypes.DEFAULT_TYPE) -> None: