Plan Mode: allow choosing a different model before implementing a plan

Summary

Plan Mode should make it possible to choose a different model before implementing an approved plan.

!Plan Mode implement prompt without model selection

What variant of Codex are you using?

App

What feature would you like to see?

When Codex finishes generating a plan and shows the Implement this plan? prompt, the user should be able to choose which model will implement that plan before submitting.

A common workflow is to use one model to reason through or draft the plan, then use a different model to perform the implementation. Today, that path is possible only indirectly: dismiss the prompt, manually switch models, then ask Codex to implement the plan. That workaround is not obvious from the prompt itself.

The Implement this plan? prompt could include an explicit model selector or otherwise make the model choice visible before implementation starts.

Additional information

This is primarily a UX discoverability/requested-control issue. The existing prompt makes the implementation action clear, but not the model that will execute it or the fact that the user can dismiss, switch models, and then request implementation manually.