Wait for Pages deployment before posting preview link

pr-preview-action posts its sticky comment as soon as the build lands
on gh-pages, ~30-60s before the GitHub Pages deployment finishes, so
the preview link 404s at first. wait-for-pages-deployment holds the
comment until the deployment is live; pages: read lets the wait poll
the Pages builds API (the explicit permissions block zeroes it
otherwise).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: pmckinney-codat

.github/workflows/pr.yml

@@ -135,6 +135,8 @@ jobs:
     permissions:
       contents: write
       pull-requests: write
+      # wait-for-pages-deployment polls the Pages builds API
+      pages: read
     steps:
       - uses: actions/checkout@v7
         with:
@@ -177,3 +179,6 @@ jobs:
         uses: rossjrw/pr-preview-action@v1
         with:
           source-dir: ./build
+          # Hold the sticky comment until the Pages deployment is live;
+          # otherwise the preview link 404s for the first minute or so
+          wait-for-pages-deployment: true