Fix Claude checkout for fork PRs

Use repository and ref from PR head to properly checkout
code from fork branches when using pull_request_target.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: a-simeshin

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

@@ -25,8 +25,9 @@ jobs:
       - name: Checkout PR code
         uses: actions/checkout@v4
         with:
-          ref: ${{ github.event.pull_request.head.sha || github.sha }}
-          fetch-depth: 1
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
+          ref: ${{ github.event.pull_request.head.ref }}
+          fetch-depth: 20
 
       - name: Run Claude Code Review
         uses: anthropics/claude-code-action@v1