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/core/tests/suite/plugins.rs

@@ -243,9 +243,9 @@ async fn explicit_plugin_mentions_inject_plugin_guidance() -> Result<()> {
 
     codex
         .submit(Op::UserInput {
-            items: vec![codex_protocol::user_input::UserInput::Text {
-                text: "Use @sample for this task.".into(),
-                text_elements: Vec::new(),
+            items: vec![codex_protocol::user_input::UserInput::Mention {
+                name: "sample".into(),
+                path: format!("plugin://{SAMPLE_PLUGIN_CONFIG_NAME}"),
             }],
             final_output_json_schema: None,
         })