Add push trigger for unstable branch in sync workflow

This update enables the workflow to trigger on pushes to the unstable branch in addition to the existing cron schedule and manual triggers. It improves automation and ensures consistency when changes are pushed to the unstable branch.

Author: urban233

.github/workflows/sync-upstream.yaml

@@ -3,7 +3,10 @@ name: Sync with Upstream (unstable → production)
 on:
   schedule:
     - cron: "0 4 * * 0"  # Every Sunday at 04:00 UTC
-  workflow_dispatch:       # Allow manual trigger from the Actions tab
+  workflow_dispatch: # Allow manual trigger from the Actions tab
+  push:
+    branches:
+      - unstable # Trigger if unstable branch is pushed to
 
 permissions:
   contents: write