Skip preview workflow for fork PRs

Fork PRs don't have access to repository secrets, causing the GCP
auth and PR comment steps to fail. Instead of failing, skip the
preview job entirely for fork PRs. The job will show as 'skipped'
rather than 'failed', allowing fork PRs to be merged based on
the CI check alone.

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.github/workflows/preview.yml

@@ -16,6 +16,8 @@ jobs:
   preview:
     name: Preview Changes
     runs-on: ubuntu-latest
+    # Skip preview for fork PRs - they don't have access to secrets
+    if: github.event.pull_request.head.repo.full_name == github.repository
     environment: production
     steps:
       - name: Checkout code