refactor(channels): abstract login() into BaseChannel, unify CLI commands

Move channel-specific login logic from CLI into each channel class via a
new `login(force=False)` method on BaseChannel. The `channels login <name>`
command now dynamically loads the channel and calls its login() method.

- WeixinChannel.login(): calls existing _qr_login(), with force to clear saved token
- WhatsAppChannel.login(): sets up bridge and spawns npm process for QR login
- CLI no longer contains duplicate login logic per channel
- Update CHANNEL_PLUGIN_GUIDE to document the login() hook

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nanobot/channels/base.py

@@ -49,6 +49,18 @@ class BaseChannel(ABC):
             logger.warning("{}: audio transcription failed: {}", self.name, e)
             return ""
 
+    async def login(self, force: bool = False) -> bool:
+        """
+        Perform channel-specific interactive login (e.g. QR code scan).
+
+        Args:
+            force: If True, ignore existing credentials and force re-authentication.
+
+        Returns True if already authenticated or login succeeds.
+        Override in subclasses that support interactive login.
+        """
+        return True
+
     @abstractmethod
     async def start(self) -> None:
         """