[extensions] Add extension management install command (#6703)

packages/cli/src/commands/extensions.tsx

@@ -0,0 +1,22 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { CommandModule } from 'yargs';
+import { installCommand } from './extensions/install.js';
+
+export const extensionsCommand: CommandModule = {
+  command: 'extensions <command>',
+  describe: 'Manage Gemini CLI extensions.',
+  builder: (yargs) =>
+    yargs
+      .command(installCommand)
+      .demandCommand(1, 'You need at least one command before continuing.')
+      .version(false),
+  handler: () => {
+    // This handler is not called when a subcommand is provided.
+    // Yargs will show the help menu.
+  },
+};