feat(workflow): trigger label sync on main branch

Ensure the label synchronization workflow is executed on pushes
to the main branch, enhancing label consistency and reducing
manual updates.

.github/workflows/sync-labels.yml

@@ -2,6 +2,8 @@ name: Sync Labels
 
 on:
   push:
+    branches:
+      - 'main'
     paths:
       - '.github/labels.json'
   workflow_dispatch: