fix(ci): pin Claude review checkout to PR head sha

Prevents `git fetch origin pull/<N>/head:main` failures on fork PRs
whose head branch is also named `main` — using a SHA puts the runner
in detached HEAD so the main ref is free for the action's internal fetch.

.github/workflows/claude.yml

@@ -44,6 +44,7 @@ jobs:
         uses: actions/checkout@v6
         with:
           fetch-depth: 1
+          ref: ${{ github.event.pull_request.head.sha || github.sha }}
 
       - name: Run Claude (review only)
         uses: anthropics/claude-code-action@v1