feat(cli): support Open Plugin (plugin.json) manifest standard

Fixes https://github.com/google-gemini/maintainers-gemini-cli/issues/1597

packages/cli/src/config/extensions/variableSchema.ts

@@ -25,6 +25,10 @@ export const VARIABLE_SCHEMA = {
     type: 'string',
     description: 'The path of the extension in the filesystem.',
   },
+  PLUGIN_ROOT: {
+    type: 'string',
+    description: 'The root path of the plugin (alias for extensionPath).',
+  },
   workspacePath: {
     type: 'string',
     description: 'The absolute path of the current workspace.',