Add a script for cleaning up stale copybara PRs

This was tested locally over a smaller set of PRs, resulting in:
https://github.com/protocolbuffers/protobuf/pull/13557

PiperOrigin-RevId: 560124506

.github/workflows/janitor.yml

@@ -0,0 +1,23 @@
+name: Protobuf Janitor
+
+on:
+  schedule:
+    # Run daily at 10 AM UTC (2 AM PDT)
+    - cron: 0 10 * * *
+  workflow_dispatch:
+
+jobs:
+  stale-prs:
+    - name: Close Stale Copybara PRs
+      runs-on: ubuntu-latest
+      run: |
+        set -ex
+        STALE_PRS=$(gh pr list --author "app/copybara-service" --limit 500 \
+            --json "number" --search "updated:<=$(date --date="-7 day" +%F)" \
+          | jq ".[].number")
+        for pr in $(STALE_PRS); do
+          echo "Closing #$pr..."
+          gh pr close --comment "Auto-closing Copybara pull request" --delete-branch "$pr"
+        done
+      env:
+        GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}