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-site/shortcuts.md

@@ -138,6 +138,8 @@ The plugin's `v` replaces mpv's own subtitle visibility toggle. When the overlay
 }
 ```
 
+To run a [plugin](/plugins) action, use `["__plugin-action:<plugin id>/<action>"]` as the command.
+
 Mouse button names are `MBTN_LEFT`, `MBTN_MID`, `MBTN_RIGHT`, `MBTN_BACK`, and `MBTN_FORWARD`. Scroll wheel names are `WHEEL_UP`, `WHEEL_DOWN`, `WHEEL_LEFT`, and `WHEEL_RIGHT`. See [keybindings](/configuration#keybindings) and [shortcuts configuration](/configuration#shortcuts-configuration) in the config reference.
 
 ## Automatic mpv bindings