Add cleanup-pr-previews workflow

.github/workflows/cleanup-pr-previews.yml

@@ -0,0 +1,21 @@
+# Housekeeping half of the PR-preview family, delegated to the reusable
+# workflow in Morrison-Lab/gha. Periodically deletes gh-pages preview directories
+# for PRs that are no longer open, and (with compact-history) orphan-squashes
+# gh-pages to a single commit so the deleted snapshots stop bloating the repo.
+# The calling job grants contents: write so the reusable workflow can commit the
+# deletions / force-push the compacted branch back to gh-pages.
+name: Clean up PR Previews
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '0 0 * * 0' # Weekly on Sunday at midnight UTC
+
+jobs:
+  cleanup:
+    permissions:
+      contents: write
+      pull-requests: read
+    uses: Morrison-Lab/gha/.github/workflows/cleanup-pr-previews.yml@v2
+    with:
+      compact-history: true