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/public/config.example.jsonc

@@ -218,6 +218,14 @@
     "overlayPosition": "top-right" // Position for in-overlay notification cards. Values: top-left | top | top-right
   }, // Overlay notification display behavior.
 
+  // ==========================================
+  // Plugins
+  // JavaScript plugins loaded from <config dir>/plugins/<id>/ at startup.
+  // Plugins run inside SubMiner with full access to your system. Only enable code you trust.
+  // Restart SubMiner after changing plugins.
+  // ==========================================
+  "plugins": {}, // Plugins to load at startup, keyed by directory name under <config dir>/plugins/. Each entry is { "enabled": true, "settings": { ... } }; settings are passed to the plugin as ctx.settings.
+
   // ==========================================
   // Keyboard Shortcuts
   // Overlay keyboard shortcuts. Set a shortcut to null to disable.