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/lib.rs

@@ -74,6 +74,7 @@ pub use mention_syntax::PLUGIN_TEXT_MENTION_SIGIL;
 pub use mention_syntax::TOOL_MENTION_SIGIL;
 pub use utils::path_utils;
 pub(crate) mod plugins;
+pub use plugins::plugins_manager_for_config;
 #[doc(hidden)]
 pub(crate) mod prompt_debug;
 #[doc(hidden)]