Add gemini extensions link command (#7241)

Co-authored-by: Bryan Morgan <bryanmorgan@google.com>

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

@@ -0,0 +1,51 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { CommandModule } from 'yargs';
+import {
+  installExtension,
+  type ExtensionInstallMetadata,
+} from '../../config/extension.js';
+
+import { getErrorMessage } from '../../utils/errors.js';
+
+interface InstallArgs {
+  path: string;
+}
+
+export async function handleLink(args: InstallArgs) {
+  try {
+    const installMetadata: ExtensionInstallMetadata = {
+      source: args.path,
+      type: 'link',
+    };
+    const extensionName = await installExtension(installMetadata);
+    console.log(
+      `Extension "${extensionName}" linked successfully and enabled.`,
+    );
+  } catch (error) {
+    console.error(getErrorMessage(error));
+    process.exit(1);
+  }
+}
+
+export const linkCommand: CommandModule = {
+  command: 'link <path>',
+  describe:
+    'Links an extension from a local path. Updates made to the local path will always be reflected.',
+  builder: (yargs) =>
+    yargs
+      .positional('path', {
+        describe: 'The name of the extension to link.',
+        type: 'string',
+      })
+      .check((_) => true),
+  handler: async (argv) => {
+    await handleLink({
+      path: argv['path'] as string,
+    });
+  },
+};