fix(agents): reload empty agent definitions in menu

## Bug 详情
生产构建后执行交互式 /agents 时，菜单可能从 AppState 读到空的 agentDefinitions，导致显示 No agents found；同一环境下 bun run dev 可以正常显示 agents。

## 根因
生产 bundle 中底层 agent loader 和内置 agents 均可用，但 /agents 动态菜单只消费启动时注入的 AppState.agentDefinitions。构建产物复现时该状态可能为空，菜单没有现场刷新兜底。

## 修复方案
在 AgentsMenu 打开时检测 allAgents 和 activeAgents 是否同时为空；为空时基于当前 cwd 调用 getAgentDefinitionsWithOverrides 重新加载并写回 AppState，正常已有 agents 的路径直接跳过。

## 变更要点
- 为 AgentsMenu 增加空 agentDefinitions 的 fallback reload
- 使用取消标记避免组件卸载后继续写状态
- 保持现有编辑、删除和正常 dev 路径不变

## 自测
- bun run build
- ./dist/cli-bun.js agents 可列出 27 active agents
- bun run typecheck 仍存在仓库既有类型错误，未新增 AgentsMenu 相关错误

src/components/agents/AgentsMenu.tsx

@@ -1,6 +1,6 @@
 import chalk from 'chalk';
 import * as React from 'react';
-import { useCallback, useMemo, useState } from 'react';
+import { useCallback, useEffect, useMemo, useState } from 'react';
 import type { SettingSource } from 'src/utils/settings/constants.js';
 import type { CommandResultDisplay } from '../../commands.js';
 import { useExitOnCtrlCDWithKeybindings } from '../../hooks/useExitOnCtrlCDWithKeybindings.js';
@@ -15,7 +15,9 @@ import {
 import {
   type AgentDefinition,
   getActiveAgentsFromList,
+  getAgentDefinitionsWithOverrides,
 } from '@claude-code-best/builtin-tools/tools/AgentTool/loadAgentsDir.js';
+import { getCwd } from '../../utils/cwd.js';
 import { toError } from '../../utils/errors.js';
 import { logError } from '../../utils/log.js';
 import { Select } from '../CustomSelect/select.js';
@@ -50,6 +52,28 @@ export function AgentsMenu({ tools, onExit }: Props): React.ReactNode {
 
   useExitOnCtrlCDWithKeybindings();
 
+  useEffect(() => {
+    if (allAgents.length > 0 || agents.length > 0) return;
+
+    let cancelled = false;
+    void getAgentDefinitionsWithOverrides(getCwd())
+      .then(freshAgentDefinitions => {
+        if (cancelled) return;
+        if (freshAgentDefinitions.allAgents.length === 0 && freshAgentDefinitions.activeAgents.length === 0) return;
+        setAppState(state => ({
+          ...state,
+          agentDefinitions: freshAgentDefinitions,
+        }));
+      })
+      .catch(error => {
+        logError(toError(error));
+      });
+
+    return () => {
+      cancelled = true;
+    };
+  }, [allAgents.length, agents.length, setAppState]);
+
   const agentsBySource: Record<SettingSource | 'all' | 'built-in' | 'plugin', AgentDefinition[]> = useMemo(
     () => ({
       'built-in': allAgents.filter(a => a.source === 'built-in'),