ci: pin review responder checkout to SHA

Author: FutureUnreal

.github/workflows/claude-review-responder.yml

@@ -23,7 +23,7 @@ jobs:
         uses: actions/checkout@v5
         with:
           fetch-depth: 0
-          ref: ${{ github.event.pull_request.head.ref }}
+          ref: ${{ github.event.pull_request.head.sha }}
 
       - name: Verify PR branch matches reviewed SHA
         run: |