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

src/config/definitions/defaults-core.ts

@@ -17,6 +17,7 @@ export const CORE_DEFAULT_CONFIG: Pick<
   | 'startupWarmups'
   | 'updates'
   | 'notifications'
+  | 'plugins'
   | 'auto_start_overlay'
 > = {
   dictionaryBackend: 'yomitan',
@@ -145,5 +146,6 @@ export const CORE_DEFAULT_CONFIG: Pick<
   notifications: {
     overlayPosition: 'top-right',
   },
+  plugins: {},
   auto_start_overlay: true,
 };