Update format.yml

Author: neetcode-gh

.github/workflows/format.yml

@@ -13,11 +13,11 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - name: Checkout PR HEAD
+      - name: Checkout PR HEAD (PR or fallback)
         uses: actions/checkout@v4
         with:
-          repository: ${{ github.event.pull_request.head.repo.full_name }}
-          ref: ${{ github.event.pull_request.head.sha }}
+          repository: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name || github.repository }}
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           fetch-depth: 0
 
       - name: Detect fork vs same-repo