Add concurrency for agent PR review workflow

.github/workflows/agent-pr-review.yml

@@ -8,6 +8,10 @@ permissions:
   contents: write
   pull-requests: write
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
 jobs:
   agent-review:
     runs-on: ubuntu-latest