CI: convert PRs to draft after review

.github/workflows/pr_draft.yaml

@@ -0,0 +1,15 @@
+name: Re-draft on review
+on:
+  pull_request_review:
+    types: [submitted]
+
+jobs:
+  redraft:
+    if: github.event.review.state == 'changes_requested'
+    permissions:
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+      - run: gh pr ready --undo "${{ github.event.pull_request.number }}"
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}