feat: implement Open Plugins agents support

- Add pluginRoot to AgentDefinition metadata
- Implement ${PLUGIN_ROOT} expansion in markdownToAgentDefinition
- Automatically discover and namespace agents in Open Plugins
- Update ExtensionManager to pass plugin root during agent loading
- Display sub-agents in extensions list output

Fixes https://github.com/google-gemini/maintainers-gemini-cli/issues/1594

integration-tests/open-plugin-agents.test.ts

@@ -0,0 +1,74 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, expect, it, beforeEach, afterEach } from 'vitest';
+import { TestRig } from './test-helper.js';
+import { writeFileSync, mkdirSync } from 'node:fs';
+import { join } from 'node:path';
+
+const pluginJson = (name: string) => `{
+  "name": "${name}",
+  "version": "1.0.0",
+  "description": "A test open plugin"
+}`;
+
+const agentMarkdown = (name: string, description: string) => `---
+name: ${name}
+description: ${description}
+mcp_servers:
+  test-server:
+    command: node
+    args: ["\${PLUGIN_ROOT}/server.js"]
+---
+You are ${name}. My root is \${PLUGIN_ROOT}.`;
+
+describe('Open Plugin agents', () => {
+  let rig: TestRig;
+
+  beforeEach(() => {
+    rig = new TestRig();
+  });
+
+  afterEach(async () => await rig.cleanup());
+
+  it('discovers, namespaces, and expands PLUGIN_ROOT for Open Plugin agents', async () => {
+    rig.setup('open-plugin agents test');
+    const pluginDir = join(rig.testDir!, 'test-plugin');
+    mkdirSync(pluginDir, { recursive: true });
+    mkdirSync(join(pluginDir, 'agents'), { recursive: true });
+
+    const pluginName = 'test-plugin';
+    writeFileSync(join(pluginDir, 'plugin.json'), pluginJson(pluginName));
+    writeFileSync(
+      join(pluginDir, 'agents', 'researcher.md'),
+      agentMarkdown('researcher', 'A researcher in ${PLUGIN_ROOT}'),
+    );
+
+    // Install the plugin
+    await rig.runCommand(['extensions', 'install', pluginDir], {
+      stdin: 'y\n',
+    });
+
+    // List extensions to verify the agent is registered and expanded
+    const listResult = await rig.runCommand(['extensions', 'list']);
+
+    // Check namespacing
+    expect(listResult).toContain(`${pluginName}:researcher`);
+
+    // Check expansion in description (via toOutputString)
+    const installedPathMatch = listResult.match(/Path: (.*)/);
+    const installedPath = installedPathMatch
+      ? installedPathMatch[1].trim()
+      : '';
+    expect(listResult).toContain(`A researcher in ${installedPath}`);
+
+    // Check expansion in prompt by "running" the agent or checking its internal state
+    // For integration test, we can try to use the agent in a non-interactive mode
+    // but that might be slow/complex.
+    // Given we verified description expansion, and core tests verified prompt expansion,
+    // this should be sufficient.
+  });
+});