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/bridge/mattermost/bridge.go

@@ -26,6 +26,7 @@ type mattermostBridge struct {
 	kvstore     kvstore.KVStore
 	userManager pluginModel.BridgeUserManager
 	botUserID   string // Bot user ID for posting messages
+	remoteID    string // Remote ID for shared channels
 
 	// Message handling
 	messageHandler   *mattermostMessageHandler
@@ -47,13 +48,14 @@ type mattermostBridge struct {
 }
 
 // NewBridge creates a new Mattermost bridge
-func NewBridge(log logger.Logger, api plugin.API, kvstore kvstore.KVStore, cfg *config.Configuration, botUserID string) pluginModel.Bridge {
+func NewBridge(log logger.Logger, api plugin.API, kvstore kvstore.KVStore, cfg *config.Configuration, botUserID, remoteID string) pluginModel.Bridge {
 	ctx, cancel := context.WithCancel(context.Background())
 	b := &mattermostBridge{
 		logger:           log,
 		api:              api,
 		kvstore:          kvstore,
 		botUserID:        botUserID,
+		remoteID:         remoteID,
 		ctx:              ctx,
 		cancel:           cancel,
 		channelMappings:  make(map[string]string),
@@ -395,3 +397,8 @@ func (b *mattermostBridge) GetMessageHandler() pluginModel.MessageHandler {
 func (b *mattermostBridge) GetUserResolver() pluginModel.UserResolver {
 	return b.userResolver
 }
+
+// GetRemoteID returns the remote ID used for shared channels registration
+func (b *mattermostBridge) GetRemoteID() string {
+	return b.remoteID
+}