[codex] Return workspace directory installed plugins (#27098)

## Summary

- return installed `workspace-directory` remote plugins by default in
`plugin/installed`
- keep shared-with-me installed plugins gated behind `plugin_sharing`
- filter remote installed plugin marketplaces by canonical marketplace
name instead of coarse workspace scope

## Validation

- `just fmt`
- `just test -p codex-core-plugins`
- `just test -p codex-app-server`
- `just fix -p codex-core-plugins`
- `just fix -p codex-app-server`
- `$xin-build` targeted verification:
- `just test -p codex-core-plugins
build_remote_installed_plugin_marketplaces_from_cache_filters_by_marketplace_name`
- `just test -p codex-app-server
plugin_installed_includes_workspace_directory_without_plugin_sharing`
- `just test -p codex-app-server
plugin_installed_includes_remote_shared_with_me_plugins`
- `just test -p codex-app-server
plugin_list_omits_shared_with_me_kind_when_plugin_sharing_disabled`

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

@@ -7,7 +7,7 @@ use crate::plugins::test_support::write_plugins_feature_config;
 use codex_core_plugins::OPENAI_BUNDLED_MARKETPLACE_NAME;
 use codex_core_plugins::PluginInstallRequest;
 use codex_core_plugins::PluginsManager;
-use codex_core_plugins::remote::RemotePluginScope;
+use codex_core_plugins::remote::REMOTE_GLOBAL_MARKETPLACE_NAME;
 use codex_core_plugins::remote::RemotePluginServiceConfig;
 use codex_core_plugins::remote::fetch_and_cache_global_remote_plugin_catalog;
 use codex_core_plugins::startup_sync::curated_plugins_repo_path;
@@ -362,7 +362,7 @@ remote_plugin = true
         .build_and_cache_remote_installed_plugin_marketplaces(
             &config.plugins_config_input(),
             Some(&auth),
-            &[RemotePluginScope::Global],
+            &[REMOTE_GLOBAL_MARKETPLACE_NAME],
             /*on_effective_plugins_changed*/ None,
         )
         .await