Address review: fix auto-clang-format checkout for fork PRs

Use github.event.pull_request.head.repo.full_name and
github.event.pull_request.head.ref to correctly check out the
PR branch when the PR originates from a fork repository.

Requested by @lefticus in PR #691 review.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/auto-clang-format.yml

@@ -8,8 +8,9 @@ jobs:
     steps:
     - uses: actions/checkout@v6
       with:
-        ref: ${{ github.head_ref || github.ref_name }}
-        fetch-depth: 0    
+        repository: ${{ github.event.pull_request.head.repo.full_name }}
+        ref: ${{ github.event.pull_request.head.ref }}
+        fetch-depth: 0
     - uses: DoozyX/clang-format-lint-action@v0.20
       with:
         source: '.'