[Feature] 自定义子代理模型及自定义勾选思考预

[duoyuo]

Affected area

Desktop UI (agent-gui / React)

Problem statement (what should this solve)

当前子代理使用的模型是主会话的，无法设置使用其它模型作为子代理
有些模型思考等级获取的不完整，漏了一些，实际上是可以使用的，比如有的模型只获得到了低和最高，但是其它等级思考预算如极限、中等都可以使用

Proposed behavior

添加子代理设置界面
允许自定在模型列表的模型设置里自定义勾选思考等级，类似workbuddy的自定义模型的思考等级设置

Estimated change scope

No response

Alternatives considered

No response

Pre-submit checklist

• I searched existing issues and pull requests and found no duplicates.
• This proposal is focused on a single feature or improvement.
• I understand a PR should come after this issue is confirmed by maintainers, otherwise it will be converted to draft.