Install the referenced plugins into $CONFIG_DIR/plugins and add them to
the marketplace file in `~/.agents/plugins/marketplace.json`. The
plugins are then enabled in `$CONFIG_DIR/config.toml`.
Also add support for custom plugin marketplaces via the `marketplaces`
option. Each marketplace entry is a local directory (or fetched package)
that Codex can browse to discover additional plugins. Marketplaces are
registered in `$CONFIG_DIR/config.toml` under [marketplaces.<name>] with
`source_type = "local"` pointing at the provided path. Setting either
`plugins` or `marketplaces` automatically enables the `features.plugins`
flag in the generated config.
Warn that if a derivation is being used as the source of the plugin, the
name of the derivation needs to match the plugin name in the manifest.
