Fix Claude review workflow checkout ref (#5874)

By default, in this case the checkout action will checkout to github's
temporary merge base ref, which may include changes from the base branch
when the base branch moves.

This happened in this review:
#5866 (review)

To prevent this, the PR's HEAD SHA was specified to be used
specifically, keeping claude's reviews only looking at that PR's branch.

Author: benceruleanlu

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

@@ -53,6 +53,7 @@ jobs:
         uses: actions/checkout@v5
         with:
           fetch-depth: 0
+          ref: refs/pull/${{ github.event.pull_request.number }}/head
 
       - name: Install pnpm
         uses: pnpm/action-setup@v4
@@ -86,4 +87,4 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           COMMIT_SHA: ${{ github.event.pull_request.head.sha }}
           BASE_SHA: ${{ github.event.pull_request.base.sha }}
-          REPOSITORY: ${{ github.repository }}
+          REPOSITORY: ${{ github.repository }}