ci: Fix Claude action PR number (#263)

Use the correct event property for referencing the PR number.

See:

- #254 (comment)
- https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#issue_comment-on-issues-only-or-pull-requests-only

Author: dcalhoun

.github/workflows/claude-review.yml

@@ -26,7 +26,7 @@ jobs:
                   track_progress: true
                   prompt: |
                       REPO: ${{ github.repository }}
-                      PR NUMBER: ${{ github.event.pull_request.number }}
+                      PR NUMBER: ${{ github.event.issue.number }}
 
                       Perform a comprehensive code review with the following focus areas: