Added a new top-level `codex marketplace add` command for installing
plugin marketplaces into Codex’s local marketplace cache.
This change adds source parsing for local directories, GitHub shorthand,
and git URLs, supports optional `--ref` and git-only `--sparse` checkout
paths, stages the source in a temp directory, validates the marketplace
manifest, and installs it under
`$CODEX_HOME/marketplaces/<marketplace-name>`
Included tests cover local install behavior in the CLI and marketplace
discovery from installed roots in core. Scoped formatting and fix passes
were run, and targeted CLI/core tests passed.
