[codex] Refactor marketplace add into shared core flow (#17717)

## Summary

Move `codex marketplace add` onto a shared core implementation so the
CLI and app-server path can use one source of truth.

This change:
- adds shared marketplace-add orchestration in `codex-core`
- switches the CLI command to call that shared implementation
- removes duplicated CLI-only marketplace add helpers
- preserves focused parser and add-path coverage while moving the shared
behavior into core tests

## Why

The new `marketplace/add` RPC should reuse the same underlying
marketplace-add flow as the CLI. This refactor lands that consolidation
first so the follow-up app-server PR can be mostly protocol and handler
wiring.

## Validation

- `cargo test -p codex-core marketplace_add`
- `cargo test -p codex-cli marketplace_cmd`
- `just fix -p codex-core`
- `just fix -p codex-cli`
- `just fmt`

codex-rs/core/src/plugins/mod.rs

@@ -6,6 +6,7 @@ mod installed_marketplaces;
 mod manager;
 mod manifest;
 mod marketplace;
+mod marketplace_add;
 mod mentions;
 mod remote;
 mod render;
@@ -58,6 +59,10 @@ pub use marketplace::MarketplacePluginInstallPolicy;
 pub use marketplace::MarketplacePluginPolicy;
 pub use marketplace::MarketplacePluginSource;
 pub use marketplace::validate_marketplace_root;
+pub use marketplace_add::MarketplaceAddError;
+pub use marketplace_add::MarketplaceAddOutcome;
+pub use marketplace_add::MarketplaceAddRequest;
+pub use marketplace_add::add_marketplace;
 pub use remote::RemotePluginFetchError;
 pub use remote::fetch_remote_featured_plugin_ids;
 pub(crate) use render::render_explicit_plugin_instructions;