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/app-server-protocol/schema/json/EventMsg.json

@@ -6281,7 +6281,7 @@
           "type": "object"
         },
         {
-          "description": "Explicit mention selected by the user (name + app://connector id).",
+          "description": "Explicit structured mention selected by the user.\n\n`path` identifies the exact mention target, for example `app://<connector-id>` or `plugin://<plugin-name>@<marketplace-name>`.",
           "properties": {
             "name": {
               "type": "string"