feat: structured plugin parsing (#13711)

#### What

Add structured `@plugin` parsing and TUI support for plugin mentions.

- Core: switch from plain-text `@display_name` parsing to structured
`plugin://...` mentions via `UserInput::Mention` and
`[$...](plugin://...)` links in text, same pattern as apps/skills.
- TUI: add plugin mention popup, autocomplete, and chips when typing
`$`. Load plugin capability summaries and feed them into the composer;
plugin mentions appear alongside skills and apps.
- Generalize mention parsing to a sigil parameter, still defaults to `$`

<img width="797" height="119" alt="image"
src="https://github.com/user-attachments/assets/f0fe2658-d908-4927-9139-73f850805ceb"
/>

Builds on #13510. Currently clients have to build their own `id` via
`plugin@marketplace` and filter plugins to show by `enabled`, but we
will add `id` and `available` as fields returned from `plugin/list`
soon.

####Tests

Added tests, verified locally.

codex-rs/tui/src/bottom_pane/mod.rs

@@ -28,6 +28,7 @@ use crate::render::renderable::RenderableItem;
 use crate::tui::FrameRequester;
 use bottom_pane_view::BottomPaneView;
 use codex_core::features::Features;
+use codex_core::plugins::PluginCapabilitySummary;
 use codex_core::skills::model::SkillMetadata;
 use codex_file_search::FileMatch;
 use codex_protocol::request_user_input::RequestUserInputEvent;
@@ -254,6 +255,11 @@ impl BottomPane {
         self.request_redraw();
     }
 
+    pub fn set_plugin_mentions(&mut self, plugins: Option<Vec<PluginCapabilitySummary>>) {
+        self.composer.set_plugin_mentions(plugins);
+        self.request_redraw();
+    }
+
     pub fn take_mention_bindings(&mut self) -> Vec<MentionBinding> {
         self.composer.take_mention_bindings()
     }
@@ -333,6 +339,10 @@ impl BottomPane {
         self.composer.skills()
     }
 
+    pub fn plugins(&self) -> Option<&Vec<PluginCapabilitySummary>> {
+        self.composer.plugins()
+    }
+
     #[cfg(test)]
     pub(crate) fn context_window_percent(&self) -> Option<i64> {
         self.context_window_percent