Add collection browsing and metadata discovery commands

[andreagrandi]

Summary

Adds read-only collection commands so users and agents can browse Metabase collections and discover their contents. Closes #13.

Commands

• collection list — list all collections
• collection get <id> — collection details (accepts root, case-insensitive)
• collection items <id> — cards, dashboards, and nested collections inside a collection, with --models filtering

Notes

• Collection.ID is typed as any because the Metabase API genuinely returns the root collection with the string id "root" alongside numeric ids (confirmed via smoke testing). JSON output stays natural — numbers as numbers, "root" as a string.
• Collection lookups use the typed mberr.RequestError / ResourceCollection path, so a 404 is classified as API_ERROR with a collection-specific suggestion.
• Both --format json and --format table are supported.

Testing

• 7 client-level tests + 2 CLI integration tests covering listing, item discovery, --models filtering, the root collection, and 404 handling.
• Full test suite passes (make test); make fmt and make vet clean.
• Smoke tested against a real Metabase instance: list, get <id>, get root, items <id> (table + JSON), items root --models dashboard, and a 404.

Changelog

Entry added under ## [Unreleased] (#13).