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/src/tools/spec.rs

@@ -684,7 +684,7 @@ fn create_collab_input_items_schema() -> JsonSchema {
             "path".to_string(),
             JsonSchema::String {
                 description: Some(
-                    "Path when type is local_image/skill, or mention target such as app://<connector-id> when type is mention."
+                    "Path when type is local_image/skill, or structured mention target such as app://<connector-id> or plugin://<plugin-name>@<marketplace-name> when type is mention."
                         .to_string(),
                 ),
             },