[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/config/src/lib.rs

@@ -63,7 +63,10 @@ pub use diagnostics::format_config_error_with_source;
 pub use diagnostics::io_error_from_config_error;
 pub use fingerprint::version_for_toml;
 pub use marketplace_edit::MarketplaceConfigUpdate;
+pub use marketplace_edit::RemoveMarketplaceConfigOutcome;
 pub use marketplace_edit::record_user_marketplace;
+pub use marketplace_edit::remove_user_marketplace;
+pub use marketplace_edit::remove_user_marketplace_config;
 pub use mcp_edit::ConfigEditsBuilder;
 pub use mcp_edit::load_global_mcp_servers;
 pub use mcp_types::AppToolApproval;