Enable "autopilot" in settings to always start the session in that mode

### Feature summary

Enable "autopilot" in settings to always start the session in that mode

### What problem are you trying to solve?

I now have to select autopilot in every session, especially now that the interactive mode does not create a PR by default, it becomes rather tedious.

### Proposed solution

Add a setting to use a default mode for new sessions.

### Workflow impact

High, it is extra clicking for every session

### Installation context

_No response_

### Additional context

_No response_

首先跟踪设置 UI 以及为新会话选择模式的代码。为 autopilot 添加持久化的默认模式设置，并验证新会话会使用该设置，而无需额外选择。

由索引模型根据 Issue 内容生成。