[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-plugins/src/discoverable.rs

@@ -14,7 +14,6 @@ use crate::PluginsConfigInput;
 use crate::PluginsManager;
 use crate::marketplace::MarketplacePluginInstallPolicy;
 use crate::remote::REMOTE_GLOBAL_MARKETPLACE_NAME;
-use crate::remote::RemotePluginScope;
 
 const TOOL_SUGGEST_DISCOVERABLE_PLUGIN_ALLOWLIST: &[&str] = &[
     "github@openai-curated",
@@ -100,7 +99,9 @@ impl PluginsManager {
             .collect::<HashSet<_>>();
         installed_app_connector_ids.extend(input.loaded_plugin_app_connector_ids.iter().cloned());
         let remote_installed_marketplaces = if input.plugins.remote_plugin_enabled {
-            self.build_remote_installed_plugin_marketplaces_from_cache(&[RemotePluginScope::Global])
+            self.build_remote_installed_plugin_marketplaces_from_cache(&[
+                REMOTE_GLOBAL_MARKETPLACE_NAME,
+            ])
         } else {
             None
         };