fix: delete stale sync branch before push in upstream sync workflow

Push fails when sync-upstream-* branch already exists from previous
failed run. Delete remote branch first to avoid conflict.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JOY

.github/workflows/sync-upstream.yml

@@ -227,6 +227,7 @@ jobs:
 
           git add -A
           git commit -m "WIP: Merge upstream $TAG (has conflicts)" --no-verify || true
+          git push origin --delete "$BRANCH" 2>/dev/null || true
           git push origin "$BRANCH"
 
           REMAINING=$(echo "${{ steps.merge.outputs.conflict_files }}" | head -20)