ci(warden): Cancel superseded PR runs (#212)

Group Warden workflow executions by pull request so a new commit cancels analysis for the previous head.

Co-authored-by: Codex <noreply@openai.com>

Author: jamieQ

.github/workflows/warden.yml

@@ -1,9 +1,14 @@
 name: Warden
 
+# Update the concurrency group below if this workflow adds non-PR triggers.
 on:
   pull_request:
     types: [opened, synchronize, reopened]
 
+concurrency:
+  group: ${{ github.workflow }}-pr-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
 jobs:
   warden:
     if: github.event.pull_request.head.repo.full_name == github.repository