Move external agent config out of core (#18850)

## Summary
- Move external agent config migration logic and tests from `codex-core`
into `app-server/src/config`.
- Keep the migration service crate-private to app-server and update the
API adapter imports.
- Remove stale core re-exports and expose only the needed marketplace
source helper.

## Testing
- `cargo test -p codex-app-server config::external_agent_config`
- `just fmt`
- `just fix -p codex-app-server`
- `just fix -p codex-core`
- `git diff --check`

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

@@ -25,7 +25,6 @@ pub use codex_plugin::validate_plugin_segment;
 pub type LoadedPlugin = codex_plugin::LoadedPlugin<McpServerConfig>;
 pub type PluginLoadOutcome = codex_plugin::PluginLoadOutcome<McpServerConfig>;
 
-pub(crate) use codex_core_plugins::marketplace::find_marketplace_manifest_path;
 pub(crate) use discoverable::list_tool_suggest_discoverable_plugins;
 pub(crate) use injection::build_plugin_injections;
 pub use installed_marketplaces::INSTALLED_MARKETPLACES_DIR;
@@ -46,13 +45,11 @@ pub use manager::PluginRemoteSyncError;
 pub use manager::PluginUninstallError;
 pub use manager::PluginsManager;
 pub use manager::RemotePluginSyncResult;
-pub(crate) use manager::configured_plugins_from_stack;
 pub use marketplace_add::MarketplaceAddError;
 pub use marketplace_add::MarketplaceAddOutcome;
 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_add::is_local_marketplace_source;
 pub use marketplace_remove::MarketplaceRemoveError;
 pub use marketplace_remove::MarketplaceRemoveOutcome;
 pub use marketplace_remove::MarketplaceRemoveRequest;