Use branch-based URL in PR preview comments

Changes PR comments to show the nicer branch-based URL:
- Before: https://ef29be34.staging-cambeerfestival.pages.dev
- After: https://claude-preview-domain-cicd-0.staging-cambeerfestival.pages.dev

The branch-based alias is more readable and matches the branch name.

Author: claude

.github/workflows/build-deploy.yml

@@ -270,7 +270,8 @@ jobs:
         uses: actions/github-script@v7
         with:
           script: |
-            const previewUrl = '${{ steps.deploy.outputs.deployment-url }}';
+            const branch = '${{ github.head_ref }}';
+            const previewUrl = `https://${branch}.staging-cambeerfestival.pages.dev`;
             const comment = `## 🚀 Cloudflare Pages Preview
 
             Your preview deployment is ready!