Address review: fix auto-clang-format detached HEAD error

actions/checkout@v6 defaults to checking out the PR merge ref in
detached HEAD state, causing EndBug/add-and-commit to fail with
"fatal: You are not currently on a branch." Fix by explicitly
checking out the PR's head branch via github.head_ref.

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: '.'