feat(plugins): add plugin windows, CLI actions, and data dirs

- Add `--plugin-action <id>/<name>` CLI flag, dispatched via runPluginAction
- Add ctx.ui.openWindow for plugin-owned BrowserWindows with sandboxed preload
- Add ctx.dataDir (`plugin-data/<id>/`), ctx.actions.run, ctx.mpv.ensureRunning
- Emit mpv end-file events to plugins with reason, entry id, and error
- Run Windows .cmd/.bat shims through cmd.exe in exec, killing the tree on abort
- Update plugin docs, usage, architecture notes, and e2e coverage

changes/plugin-system.md

@@ -1,5 +1,5 @@
 type: added
 area: plugins
 
-- Added JavaScript plugins loaded at startup from `<config dir>/plugins/<id>/` and enabled under `plugins` in the config. Plugins can listen to mpv events, run timers, register actions bound with `__plugin-action:<id>/<action>` keybindings, run programs, and show markdown results in an overlay panel.
+- Added JavaScript plugins loaded at startup from `<config dir>/plugins/<id>/` and enabled under `plugins` in the config. Plugins can listen to mpv events, run timers, register actions bound with `__plugin-action:<id>/<action>` keybindings or run with `--plugin-action`, run built-in actions, start mpv, run programs, keep state in a per-plugin data folder, and show results in an overlay panel or their own window.
 - Added an example `ai-explain` plugin that sends the current line and its neighbors to `codex exec` and shows a translation or grammar explanation.