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/manager.rs

@@ -198,11 +198,6 @@ impl PluginsManager {
         force_reload: bool,
     ) -> PluginLoadOutcome {
         if !plugins_feature_enabled_from_stack(config_layer_stack) {
-            let mut cache = match self.cache_by_cwd.write() {
-                Ok(cache) => cache,
-                Err(err) => err.into_inner(),
-            };
-            cache.insert(cwd.to_path_buf(), PluginLoadOutcome::default());
             return PluginLoadOutcome::default();
         }