mb pull: suggest available mixtapes when image not found

[bdougie]

Problem

When mb pull fails because a mixtape doesn't exist in the registry, the error is an opaque OCI manifest error:

$ mb pull base:latest
==> Pulling mixtape "base:latest"...
Error: fetching manifest for download.stereos.ai/mixtapes/base:latest: GET https://download.stereos.ai/v2/mixtapes/base/manifests/latest: MANIFEST_UNKNOWN: manifest unknown; map[Tag:latest]

This is also the default path for new users — mb init scaffolds a jcard.toml with mixtape = "base:latest", which doesn't exist in the registry. Running mb up then tells you to mb pull base:latest, which also fails.

Expected

Catch the MANIFEST_UNKNOWN error and suggest a mixtape that works:

Error: mixtape "base:latest" not found in the registry.

Available mixtapes:
  opencode-mixtape:latest

Pull one with: mb pull opencode-mixtape

Alternatively, update mb init to default to a mixtape that exists.

Reproduce

mb pull base:latest

Version

mb version
Version: v0.0.1-rc.10

[jpmcb]

The DX is worth considering: what would you think about

mb mixtape pull
mb mixtape ls

(i.e., hoisting mixtape management into its own "mixtape" subcommand?