ci: add sync-branches workflow to keep main and master in sync

.github/workflows/sync-branches.yml

@@ -0,0 +1,34 @@
+name: Sync main and master branches
+
+on:
+  push:
+    branches:
+      - main
+      - master
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          token: ${{ secrets.PAT_TOKEN }}
+
+      - name: Sync branches
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+
+          if [ "${{ github.ref }}" = "refs/heads/main" ]; then
+            echo "main was updated, syncing master..."
+            git checkout master
+            git reset --hard origin/main
+            git push origin master --force
+          elif [ "${{ github.ref }}" = "refs/heads/master" ]; then
+            echo "master was updated, syncing main..."
+            git checkout main
+            git reset --hard origin/master
+            git push origin main --force
+          fi