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/codex.rs

@@ -5129,7 +5129,7 @@ pub(crate) async fn run_turn(
         .services
         .plugins_manager
         .plugins_for_config(&turn_context.config);
-    // Plain-text @plugin mentions are resolved from the current session's
+    // Structured plugin:// mentions are resolved from the current session's
     // enabled plugins, then converted into turn-scoped guidance below.
     let mentioned_plugins =
         collect_explicit_plugin_mentions(&input, loaded_plugins.capability_summaries());
@@ -5226,7 +5226,7 @@ pub(crate) async fn run_turn(
         &available_connectors,
         &skill_name_counts_lower,
     ));
-    // Explicit @plugin mentions can make a plugin's enabled apps callable for
+    // Explicit plugin mentions can make a plugin's enabled apps callable for
     // this turn without persisting those connectors as sticky user selections.
     let mut turn_enabled_connectors = explicitly_enabled_connectors.clone();
     turn_enabled_connectors.extend(