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

packages/cli/src/config/extension-manager.ts

@@ -997,6 +997,7 @@ Would you like to attempt to install via "git clone" instead?`,
 
       const agentLoadResult = await loadAgentsFromDirectory(
         path.join(effectiveExtensionPath, 'agents'),
+        effectiveExtensionPath,
       );
       agentLoadResult.agents = agentLoadResult.agents.map((agent) => ({
         ...recursivelyHydrateStrings(agent, hydrationContext),
@@ -1184,6 +1185,12 @@ Would you like to attempt to install via "git clone" instead?`,
         output += `\n  ${skill.name}: ${skill.description}`;
       });
     }
+    if (extension.agents && extension.agents.length > 0) {
+      output += `\n Sub-agents:`;
+      extension.agents.forEach((agent) => {
+        output += `\n  ${agent.name}: ${agent.description}`;
+      });
+    }
     const resolvedSettings = extension.resolvedSettings;
     if (resolvedSettings && resolvedSettings.length > 0) {
       output += `\n Settings:`;

packages/cli/src/config/plugin.ts

@@ -9,6 +9,7 @@ import * as path from 'node:path';
 import { z } from 'zod';
 import {
   loadSkillsFromDir,
+  loadAgentsFromDirectory,
   type ExtensionInstallMetadata,
   type GeminiCLIExtension,
   type MCPServerConfig,
@@ -246,6 +247,12 @@ export async function createOpenPlugin(
     hydrationContext,
   );
 
+  const agents = await resolvePluginAgents(
+    pluginDir,
+    config.name,
+    hydrationContext,
+  );
+
   return {
     name: config.name,
     version: config.version,
@@ -264,7 +271,7 @@ export async function createOpenPlugin(
     settings: undefined,
     resolvedSettings: undefined,
     skills,
-    agents: undefined,
+    agents,
     themes: undefined,
   };
 }
@@ -290,3 +297,25 @@ async function resolvePluginSkills(
     extensionName: pluginName,
   }));
 }
+
+/**
+ * Discovers and namespaces agents for an Open Plugin.
+ */
+async function resolvePluginAgents(
+  pluginDir: string,
+  pluginName: string,
+  hydrationContext: Record<string, string>,
+): Promise<GeminiCLIExtension['agents']> {
+  const agentsDir = path.join(pluginDir, 'agents');
+  const agentLoadResult = await loadAgentsFromDirectory(agentsDir, pluginDir);
+
+  if (agentLoadResult.agents.length === 0) {
+    return undefined;
+  }
+
+  return agentLoadResult.agents.map((agent) => ({
+    ...recursivelyHydrateStrings(agent, hydrationContext),
+    name: `${pluginName}:${agent.name}`,
+    extensionName: pluginName,
+  }));
+}