feat: implement sync and sync-reset commands for shared channel management

- Add GetRemoteID() method to Bridge interface for cursor operations
- Update bridge constructors to accept and store remoteID parameter
- Implement executeSyncCommand handler for forcing shared channel sync
- Implement executeSyncResetCommand handler for resetting sync cursor
- Add command registration for 'sync' and 'sync-reset' subcommands
- Enhance command handler with direct plugin API access for shared channel operations
- Add comprehensive validation and error handling for unmapped channels
- Support both SyncSharedChannel and UpdateSharedChannelCursor API methods

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

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

server/model/bridge.go

@@ -161,6 +161,9 @@ type Bridge interface {
 	SendMessage(msg *BridgeMessage) error
 	GetMessageHandler() MessageHandler
 	GetUserResolver() UserResolver
+
+	// GetRemoteID returns the remote ID used for shared channels registration
+	GetRemoteID() string
 }
 
 // BridgeUser represents a user connected to any bridge service