add @plugin mentions (#13510)

## Note-- added plugin mentions via @, but that conflicts with file
mentions

depends and builds upon #13433.

- introduces explicit `@plugin` mentions. this injects the plugin's mcp
servers, app names, and skill name format into turn context as a dev
message.
- we do not yet have UI for these mentions, so we currently parse raw
text (as opposed to skills and apps which have UI chips, autocomplete,
etc.) this depends on a `plugins/list` app-server endpoint we can feed
the UI with, which is upcoming
- also annotate mcp and app tool descriptions with the plugin(s) they
come from. this gives the model a first class way of understanding what
tools come from which plugins, which will help implicit invocation.

### Tests
Added and updated tests, unit and integration. Also confirmed locally a
raw `@plugin` injects the dev message, and the model knows about its
apps, mcps, and skills.

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

@@ -1,9 +1,11 @@
+mod injection;
 mod manager;
 mod manifest;
 mod marketplace;
 mod render;
 mod store;
 
+pub(crate) use injection::build_plugin_injections;
 pub use manager::AppConnectorId;
 pub use manager::LoadedPlugin;
 pub use manager::PluginCapabilitySummary;
@@ -14,6 +16,7 @@ pub use manager::PluginsManager;
 pub(crate) use manager::plugin_namespace_for_skill_path;
 pub(crate) use manifest::load_plugin_manifest;
 pub(crate) use manifest::plugin_manifest_name;
+pub(crate) use render::render_explicit_plugin_instructions;
 pub(crate) use render::render_plugins_section;
 pub use store::PluginId;
 pub use store::PluginInstallResult;