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/resolve/plugins.ts

@@ -0,0 +1,30 @@
+import { ResolveContext } from './context';
+import { isObject } from './shared';
+
+/** Keeps well-formed `plugins.<id>` entries; ids are validated again when the plugin loads. */
+export function applyPluginsConfig(context: ResolveContext): void {
+  const { src, resolved, warn } = context;
+  if (src.plugins === undefined) return;
+  if (!isObject(src.plugins)) {
+    warn('plugins', src.plugins, resolved.plugins, 'Expected object keyed by plugin id.');
+    return;
+  }
+
+  for (const [id, entry] of Object.entries(src.plugins)) {
+    const path = `plugins.${id}`;
+    if (!isObject(entry)) {
+      warn(path, entry, undefined, 'Expected { "enabled": boolean, "settings"?: object }.');
+      continue;
+    }
+    if (entry.enabled !== undefined && typeof entry.enabled !== 'boolean') {
+      warn(`${path}.enabled`, entry.enabled, false, 'Expected boolean.');
+    }
+    if (entry.settings !== undefined && !isObject(entry.settings)) {
+      warn(`${path}.settings`, entry.settings, {}, 'Expected object.');
+    }
+    resolved.plugins[id] = {
+      enabled: entry.enabled === true,
+      settings: isObject(entry.settings) ? { ...entry.settings } : {},
+    };
+  }
+}