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.ts

@@ -37,6 +37,7 @@ const {
   startupWarmups,
   updates,
   notifications,
+  plugins,
   auto_start_overlay,
 } = CORE_DEFAULT_CONFIG;
 const {
@@ -74,6 +75,7 @@ export const DEFAULT_CONFIG: ResolvedConfig = {
   startupWarmups,
   updates,
   notifications,
+  plugins,
   subtitleStyle,
   subtitleSidebar,
   auto_start_overlay,