Add GitHub Actions workflow to clean up PR branch caches on close (#931)

PR merge ref caches (`refs/pull/{number}/merge`) accumulate after PRs
close and are never automatically purged, consuming repository cache
quota unnecessarily.

## Changes

- **`.github/workflows/CleanupCaches.yml`**: Workflow triggered on
`pull_request: closed` that enumerates and deletes all Actions caches
scoped to the closed PR's merge ref using native `gh cache` CLI commands
with `secrets.GITHUB_TOKEN` — no additional secrets required. Uses a
`permissions: actions: write` block to follow least-privilege security
practice. No repository checkout is required since the script only calls
the GitHub API.

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: BenjaminMichaelis <22186029+BenjaminMichaelis@users.noreply.github.com>

Author: Copilot

.github/workflows/CleanupCaches.yml

@@ -4,29 +4,27 @@ on:
     types:
       - closed
 
+permissions:
+  actions: write
+
 jobs:
   cleanup:
     runs-on: ubuntu-latest
     steps:
-      - name: Check out code
-        uses: actions/checkout@v6
-        
       - name: Cleanup
         run: |
-          gh extension install actions/gh-actions-cache
-          
           REPO=${{ github.repository }}
           BRANCH="refs/pull/${{ github.event.pull_request.number }}/merge"
 
-          echo "Fetching list of cache key"
-          cacheKeysForPR=$(gh actions-cache list -R $REPO -B $BRANCH | cut -f 1 )
+          echo "Fetching list of cache keys"
+          cacheKeysForPR=$(gh cache list -R $REPO --ref $BRANCH | cut -f 1)
 
-          ## Setting this to not fail the workflow while deleting cache keys. 
+          ## Setting this to not fail the workflow while deleting cache keys.
           set +e
           echo "Deleting caches..."
           for cacheKey in $cacheKeysForPR
           do
-              gh actions-cache delete $cacheKey -R $REPO -B $BRANCH --confirm
+              gh cache delete $cacheKey -R $REPO
           done
           echo "Done"
         env: