Default costume agents

### Describe the feature or problem you'd like to solve

It's really annoying constantly changing to my default agent every time I /clear or /new.

### Proposed solution

Being able to select the default agent we want to run new sessions.

### Example prompts or workflows

_No response_

### Additional context

_No response_

首先跟踪 /clear 和 /new 命令如何初始化会话，以及当前 agent 如何被选择。检查现有的配置或 agent 选择入口，然后验证用户选择的默认 agent 是否始终用于新会话以及清除会话之后。

由索引模型根据 Issue 内容生成。