ci: cancel superseded PR runs to free runner capacity

cancel-in-progress is gated to pull_request events so merge_group and
push runs always complete — only redundant CI from rapid PR pushes
gets cancelled.

.github/workflows/ci.yml

@@ -8,6 +8,10 @@ on:
   merge_group:
   workflow_dispatch:
 
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   format:
     runs-on: ubuntu-latest