Address review: restore ref option to fix detached HEAD in auto-clang-format

The previous commit (5fc11e6) incorrectly removed the ref option from
the checkout step. actions/checkout@v6 on pull_request events defaults
to the merge ref (detached HEAD), so ref: github.head_ref is required
for EndBug/add-and-commit to push back to the PR branch.

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

@@ -7,6 +7,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v6
+      with:
+        ref: ${{ github.head_ref }}
     - uses: DoozyX/clang-format-lint-action@v0.20
       with:
         source: '.'