dev-inf: Checkout PR head SHA for Claude Code review action

When using pull_request_target, the workflow runs in the context of the
base branch. By default, checkout merges with the target branch, which
could result in the LLM reporting on things that don't appear to
actually exist in the PR.

Explicitly set ref to the PR head SHA to analyze the actual PR code.

Release note: None
Epic: None

Author: ajstorm

.github/workflows/pr-analyzer-threestage.yml

@@ -16,6 +16,7 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v5
         with:
+          ref: ${{ github.event.pull_request.head.sha || github.ref }}
           fetch-depth: 1
 
       - name: Authenticate to Google Cloud