Choose model for accepted plan for autopilot

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

I frequently plan on a thinking model and prefer a non-thinking one for implementation

### Proposed solution

Avoid wasting 3x due to the incredibly convenient default choice :)

The proposed solution would offer (on first run):

```
1. Accept plan and build on autopilot (recommended) - Claude Opus 4.6
2. Accept plan and build on autopilot with another model
3. Exit plan mode and I will prompt myself
4. Suggest changes
```

> [!IMPORTANT]
> On **1.** the current model that would be used for autopilot should be clearly shown, since otherwise there's no way to see which one will do the coding

Upon choosing 2., the user would get the same model picker as if they had run `/model`. This selection would be persisted as a preference (for going from Plan > Autopilot).

Next time they run in plan mode, the options would now be:

```
1. Accept plan and build on autopilot (recommended) - [LAST SELECTED MODEL]
2. Accept plan and build on autopilot - Claude Opus 4.6
3. Accept plan and build on autopilot with another model
4. Exit plan mode and I will prompt myself
5. Suggest changes
```

> [!NOTE]
> Now option **2.**, just like **1** before, shows the current model being used in plan mode as a fixed choice too for convenience.

In my case, I'd likely choose my desired autopilot model once and then go from Plan > Autopilot without thinking after that.

### Example prompts or workflows

_No response_

### Additional context

_No response_

首先跟踪从 Plan 到 Autopilot 的流程以及现有的 /model picker。明确提议的选项如何显示当前模型和已保存的模型、如何持久化选择，以及从 Plan 切换到 Autopilot 时应验证哪些行为。

由索引模型根据 Issue 内容生成。