## Why
Unified `@` search could show a plugin alongside the skills it owns, creating duplicate entry points for the same capability.
## What changed
- Add a nullable `pluginId` to `SkillMetadata` returned by `skills/list` so clients can associate skills with their owning plugin without inspecting names or paths.
- Hide a plugin's owned skills from unified `@` results when that plugin is present, while retaining standalone skills and skills with unknown ownership.
- Keep `$` focused on individual skills and apps, and use it consistently as the skills-list shortcut. Preserve the legacy mention behavior when `mentions_v2` is disabled.
## Testing
- Cover plugin ownership in `skills/list` and the updated `@` and `$` composer results.
GitOrigin-RevId: e168cdff8d0c58f74ce4c228a683880a3167775b
victor-openai
5918c743f3