[extensions] Add extensions list command (#6879)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

packages/cli/src/commands/extensions.tsx

@@ -7,6 +7,7 @@
 import { CommandModule } from 'yargs';
 import { installCommand } from './extensions/install.js';
 import { uninstallCommand } from './extensions/uninstall.js';
+import { listCommand } from './extensions/list.js';
 
 export const extensionsCommand: CommandModule = {
   command: 'extensions <command>',
@@ -15,6 +16,7 @@ export const extensionsCommand: CommandModule = {
     yargs
       .command(installCommand)
       .command(uninstallCommand)
+      .command(listCommand)
       .demandCommand(1, 'You need at least one command before continuing.')
       .version(false),
   handler: () => {

packages/cli/src/commands/extensions/list.ts

@@ -0,0 +1,35 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { CommandModule } from 'yargs';
+import { loadUserExtensions, toOutputString } from '../../config/extension.js';
+
+export async function handleList() {
+  try {
+    const extensions = loadUserExtensions();
+    if (extensions.length === 0) {
+      console.log('No extensions installed.');
+      return;
+    }
+    console.log(
+      extensions
+        .map((extension, _): string => toOutputString(extension))
+        .join('\n\n'),
+    );
+  } catch (error) {
+    console.error((error as Error).message);
+    process.exit(1);
+  }
+}
+
+export const listCommand: CommandModule = {
+  command: 'list',
+  describe: 'Lists installed extensions.',
+  builder: (yargs) => yargs,
+  handler: async () => {
+    await handleList();
+  },
+};