[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 |

Start at the /chat Plan > Implementation transition and trace how the current session model and effort are selected. Done means the user can change both before implementation begins, without stopping the execution; verify the transition preserves those selections.

索引モデルが issue の本文から書いたものです。