[FEAT] Allow to switch Model / Effort before transitioning between Plan > Implementation

When switching from Plan > Implementation, there is no way to change the effort before we kick the implementation off, other than stopping the execution, change the effort and type "continue"

Would be great to be able to change the main model / effort running the session when transitioning from planning.

I usually use frontier capabilities to plan, then more balanced options to orchestrate the Implementation

---
| Field | Value |
| --- | --- |
| App version | 1.1.5 |
| OS | macOS 26.5.2 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 13 |

从 /chat Plan > Implementation 转换开始，跟踪当前会话模型和工作量的选择方式。完成标准是：用户可以在实施开始前更改这两项，而无需停止执行；验证该转换会保留这些选择。

由索引模型根据 Issue 内容生成。