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/protocol/src/user_input.rs

@@ -32,7 +32,10 @@ pub enum UserInput {
         name: String,
         path: std::path::PathBuf,
     },
-    /// Explicit mention selected by the user (name + app://connector id).
+    /// Explicit structured mention selected by the user.
+    ///
+    /// `path` identifies the exact mention target, for example
+    /// `app://<connector-id>` or `plugin://<plugin-name>@<marketplace-name>`.
     Mention { name: String, path: String },
 }