Grant pull-requests: write to staging deploy job

The github-script PR comment step needs this permission; without it
the GITHUB_TOKEN gets Resource not accessible by integration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/staging.yml

@@ -13,6 +13,8 @@ jobs:
   deploy:
     if: github.event_name == 'push'
     runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
     env:
       BRANCH: ${{ github.ref_name }}
       FLY_API_TOKEN: ${{ secrets.FLY_API_TOKEN }}