Add GitHub workflow to check for PRs with merge-blocking labels

.github/workflows/check-merge-labels.yml

@@ -0,0 +1,18 @@
+name: Check merge labels
+
+on:
+  pull_request:
+    types: [labeled, unlabeled]
+  merge_group:
+permissions: read-all
+
+jobs:
+  label_checks:
+    runs-on: ubuntu-latest
+    steps:
+      - id: manually_blocked
+        if: |
+          contains(github.event.pull_request.labels.*.name, 'do not merge')
+        run: |
+          echo "This is labeled \"Do not merge\"."
+          exit 1