ci: clean up stale GH Pages subdirectories

Author: cwillisf

.github/workflows/ghpages-cleanup.yml

@@ -0,0 +1,27 @@
+on:
+  schedule:
+    - cron: 0 0 * * 6 # midnight on Saturdays
+  workflow_dispatch:
+
+jobs:
+  cleanup:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out GH Pages branch
+        uses: actions/checkout
+        with:
+          # replace `fetch-depth` with `shallow-since` if and when actions/checkout#619 (or an equivalent) gets merged
+          fetch-depth: 0
+          ref: gh-pages
+      - name: Mark stale directories for removal
+        run: |
+          for dir in */; do
+            if [ -z "$(git log -n 1 --since "1 month ago" -- "$dir")" ]; then
+              git rm -r "$dir"
+            fi
+          done
+          git status
+      - name: Commit
+        run: "git commit -m 'chore: remove stale GH Pages branches'"
+      - name: Push
+        run: "git push"