Unify plugin skill loading with the host skill service (#37444)

## What changed

- Inject the host skill loader into `PluginsManager` so plugin discovery and agent turns use the same loading and product-policy behavior.
- Share plugin skill snapshots across those paths, preserving a consistent view of skills across workspaces.
- Apply migrated-command precedence after product filtering, allowing an eligible migrated command to replace a filtered native skill with the same name.

## Testing

- Add coverage for product-restricted plugin skills, native-versus-migrated command precedence, and the skills exposed to agent turns.

GitOrigin-RevId: f5ef0d0766ebeeb30d73ffaf044d003c2906ea4d

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

@@ -8,6 +8,11 @@ mod skill_snapshot_tests;
 #[cfg(test)]
 pub(crate) mod test_support;
 
+use crate::config::Config;
+use codex_core_plugins::PluginsManager;
+use codex_skills_extension::HostSkillsService;
+use std::sync::Arc;
+
 pub(crate) use codex_plugin::PluginCapabilitySummary;
 
 pub(crate) use discoverable::list_tool_suggest_discoverable_plugins;
@@ -19,3 +24,14 @@ pub(crate) use mentions::build_skill_name_counts;
 pub(crate) use mentions::collect_explicit_app_ids;
 pub(crate) use mentions::collect_explicit_plugin_mentions;
 pub(crate) use mentions::collect_tool_mentions_from_messages;
+
+/// Constructs a standalone plugin manager with extension-owned plugin skill loading.
+///
+/// Callers that already own a host skills service should inject that existing service instead.
+pub fn plugins_manager_for_config(config: &Config) -> PluginsManager {
+    let skill_root_loader = Arc::new(HostSkillsService::new(
+        config.codex_home.clone(),
+        /*bundled_skills_enabled*/ false,
+    ));
+    PluginsManager::new(config.codex_home.to_path_buf(), skill_root_loader)
+}