Add workflow to clean old GitHub Actions caches

Adds a scheduled GitHub Actions workflow (.github/workflows/cleanup_caches.yml) that runs weekly (and via workflow_dispatch) to delete Actions caches not accessed in the last 3 days. The job uses the gh CLI with the repository token and actions: write permission to list caches, filter by last_accessed_at against a 3-day cutoff, and delete matching cache IDs.

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