feat: implement OnSharedChannelsSyncMsg hook for bidirectional sync

- Add PublishMessage method to BridgeManager interface and implementation
- Implement OnSharedChannelsSyncMsg hook to process Mattermost shared channel sync messages
- Add processSyncPost helper to convert Mattermost posts to bridge messages
- Route sync messages from Mattermost shared channels to XMPP bridge via message bus
- Handle user resolution with fallback to API calls for missing users
- Add comprehensive logging and error handling for sync operations
- Support routing of text messages from shared channels to XMPP rooms

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

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

server/bridge/manager.go

@@ -550,3 +550,14 @@ func (m *BridgeManager) StopMessageRouting() error {
 	// Stop the message bus
 	return m.messageBus.Stop()
 }
+
+// PublishMessage publishes a message to the message bus for routing to target bridges
+func (m *BridgeManager) PublishMessage(msg *model.DirectionalMessage) error {
+	m.logger.LogDebug("Publishing message to message bus",
+		"source_bridge", msg.SourceBridge,
+		"direction", msg.Direction,
+		"target_bridges", msg.TargetBridges,
+		"message_id", msg.MessageID)
+
+	return m.messageBus.Publish(msg)
+}