feat: add /xmppbridge unmap command for channel unmapping

- Add DeleteChannelRoomMapping method to Bridge interface
- Implement channel unmapping logic in XMPP bridge (cache + KVStore removal)
- Add /xmppbridge unmap command handler with validation
- Bridge user automatically leaves XMPP room when unmapping
- Update command help text and autocomplete

🤖 Generated with [Claude Code](https://claude.ai/code)

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

server/model/bridge.go

@@ -57,6 +57,9 @@ type Bridge interface {
 	// GetChannelRoomMapping retrieves the bridge room ID for a given Mattermost channel ID.
 	GetChannelRoomMapping(channelID string) (string, error)
 
+	// DeleteChannelRoomMapping removes a mapping between a Mattermost channel ID and a bridge room ID.
+	DeleteChannelRoomMapping(channelID string) error
+
 	// IsConnected checks if the bridge is connected to the remote service.
 	IsConnected() bool
 }