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/discord.py

@@ -577,6 +577,7 @@ class DiscordChannel(BaseChannel):
                 media=media_paths,
                 metadata=metadata,
                 session_key=session_key,
+                is_dm=message.guild is None,
             )
         except Exception:
             await self._clear_reactions(channel_id)