CI: clean up stale branches periodically

.github/workflows/cleanup.yml

@@ -0,0 +1,22 @@
+name: Cleanup
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: "0 4 1 * *" # 4AM on 1st of every month
+    - cron: "0 4 15 * *" # 4AM on the 15th of every month
+jobs:
+  branches:
+    name: Cleanup old branches
+    runs-on: ubuntu-latest
+    steps:
+      - name: "Checkout"
+        uses: actions/checkout@v4
+
+      - name: "Delete old branches"
+        uses: beatlabs/delete-old-branches-action@v0.0.10
+        with:
+          repo_token: "${{ secrets.GITHUB_TOKEN }}"
+          date: "1 months ago"
+          dry_run: false
+          delete_tags: false
+          exclude_open_pr_branches: true