feat(plugins): add JavaScript plugin system with overlay panel

- Load plugins from <config dir>/plugins/<id>/ when enabled under `plugins` in config
- Expose ctx with mpv events/commands, timers, subtitle context, actions, notify, exec, and panel
- Bind plugin actions via `__plugin-action:<id>/<action>` keybindings (runPluginAction session action)
- Add plugin-panel modal with markdown rendering; closing aborts the panel signal
- Add ai-explain example plugin that queries `codex exec` for the current line
- Add e2e harness `files` option and plugin e2e scenario; document plugins in docs-site and internal docs

docs/architecture/domains.md

@@ -34,6 +34,7 @@ Read when: you need to find the owner module for a behavior or test surface
 - AniList tracking + character dictionary: `src/core/services/anilist/`, `src/main/runtime/composers/anilist-*`, `src/main/character-dictionary-runtime.ts`, `src/main/character-dictionary-runtime/`
 - TMDB live-action metadata: `src/core/services/tmdb/` (client + exact-title resolver), `src/core/services/immersion-tracker/live-action-link.ts` (links an entry to a TMDB title and merges other holders of the same title). The AniList cover-art fetcher calls the resolver as its fallback; `imm_anime.media_kind` marks the result and keeps the entry out of AniList season repair.
 - Jellyfin integration: `src/core/services/jellyfin*.ts`, `src/main/runtime/composers/jellyfin-*`
+- User plugins: `src/core/services/plugins/` (host, `ctx`, `exec`), `src/main/runtime/plugin-panel-runtime.ts` (panel modal), `src/renderer/modals/plugin-panel*.ts`; example in `examples/plugins/`
 - Window trackers: `src/window-trackers/`
 - Stats HTTP app: `src/core/services/stats-server.ts`, with route groups and shared route support
   in `src/core/services/stats-server/`