[feat] 智能体配置需要持久化

### Before you start

- [x] I searched [existing issues](https://github.com/bytedance/deer-flow/issues?q=is%3Aissue) and this is not a duplicate.

### Problem / motivation

md文件配置存agent不方便接口调用

### Proposed solution

自定义agent可以给第三方通过智能体名称访问

### Affected area(s)

Backend API (gateway / endpoints / SSE)

### Alternatives considered

_No response_

### Additional context

_No response_

No files or tests are named. Start by inspecting the backend gateway, endpoints, and SSE area to understand how agent configuration is currently loaded; clarify the persistence mechanism and the API contract for retrieving custom agents by name before implementation.

Written by the indexing model from the issue text.