[codex] Add marketplace remove command and shared logic (#17752)

## Summary

Move the marketplace remove implementation into shared core logic so
both the CLI command and follow-up app-server RPC can reuse the same
behavior.

This change:
- adds a shared `codex_core::plugins::remove_marketplace(...)` flow
- moves validation, config removal, and installed-root deletion out of
the CLI
- keeps the CLI as a thin wrapper over the shared implementation
- adds focused core coverage for the shared remove path

## Validation

- `just fmt`
- focused local coverage for the shared remove path
- heavier follow-up validation deferred to stacked PR CI

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

@@ -5,6 +5,7 @@ mod injection;
 mod installed_marketplaces;
 mod manager;
 mod marketplace_add;
+mod marketplace_remove;
 mod mentions;
 mod render;
 mod startup_sync;
@@ -52,6 +53,10 @@ pub use marketplace_add::MarketplaceAddRequest;
 pub use marketplace_add::add_marketplace;
 pub(crate) use marketplace_add::is_local_marketplace_source;
 pub(crate) use marketplace_add::parse_marketplace_source;
+pub use marketplace_remove::MarketplaceRemoveError;
+pub use marketplace_remove::MarketplaceRemoveOutcome;
+pub use marketplace_remove::MarketplaceRemoveRequest;
+pub use marketplace_remove::remove_marketplace;
 pub(crate) use render::render_explicit_plugin_instructions;
 pub(crate) use render::render_plugins_section;
 pub(crate) use startup_sync::curated_plugins_repo_path;