Add GitHub Actions workflow to clean up PR branch caches on close

[Copilot]

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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[Copilot]

Copilot wasn't able to review any files in this pull request.

[BenjaminMichaelis]

@copilot I don't see the changed file

[Copilot]

@copilot I don't see the changed file

The file was identical to what was already on main, so nothing showed as changed. Fixed in ec184db with these improvements:

• Removed the unnecessary actions/checkout@v6 step (no repo files are needed)
• Replaced the archived actions/gh-actions-cache extension with native gh cache commands (built into gh CLI ≥2.44.0, which ships on ubuntu-latest)
• Added a permissions: actions: write block to follow least-privilege security practice