Enable setting to configure "agent merge" options

### Feature summary

Make Agent merge easier to configure

### What problem are you trying to solve?

Right now I hae to configure Agent merge __every single time__ I select it:

I'd prefer a setting to configure a default.

### Proposed solution

Setting to configure a default

### Workflow impact

High, less clicking (now for every session)

### Installation context

_No response_

### Additional context

_No response_

该 issue 未指定文件、测试或入口点。首先定位 Agent merge selection flow 以及应用中现有的设置模式。当用户可以为 Agent merge options 配置默认值，并且该选择会在后续会话中自动应用时，即视为完成。

由索引模型根据 Issue 内容生成。