Add Vagrant box repository proxy support #265

Vagrant is listed as incomplete in the supported registries table. Add read-only caching proxy support for Vagrant box repositories.

The handler should proxy box catalog metadata, rewrite provider download URLs to the proxy, follow upstream redirects, and cache .box files by box, version, provider, and architecture. Verify downloaded files against the checksum and checksum type advertised by the catalog.

Support configurable upstream catalogs, including custom catalogs and Vagrant Cloud, with configured credentials. Catalog metadata should follow the existing TTL and stale-cache behavior. Cached boxes should remain available when the upstream is unavailable.

Cover the Vagrant client flow and protocol behavior with tests:

• catalog metadata and download URL rewriting
• version, provider, and architecture selection
• checksum verification
• redirects, range requests, cache hits, and offline reads
• private upstream authentication
• README and configuration instructions for Vagrant