[FEATURE] Add the ability to select the model separately for planning and execution

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

It is common knowledge that Opus is better for planning, while execution is a much easier task and can/should be handled by a faster model like Sonnet. Inability to select a model per task creates an awkward user experience.

Proposed Solution

Whatever fits the vision better:

• an option in Settings to be able to assign a default model per mode (auto-accept / plan)
• add a configuration to settings.json to allow assigning a default model per mode (auto-accept / plan)
• extra option in the final stage of the plan to offer "auto-accept with Sonnet 4.6", etc.

Alternative Solutions

Awkward workflow interruption with escaping from the plan and coming back asking to present it again.

Priority

High - Significant impact on productivity

Feature Category

Configuration and settings

Use Case Example

Example:
The user uses Opus for planning. Once planning is completed, the Claude Code asks nicely with a list of options to select from, like "accept", "clear context and accept", etc. To be able to optimize the cost and performance, the user has to escape, change the model, and he is no longer presented with those options, especially the best one, which clears context. So the user has to ask Claude Code to present the plan again, which is a wasted step and a source of friction.

While Claude Code is capable of changing the mode on user request (which is awesome), it is unable to change the model yet, so it could use some improvement.

Additional Context

@bcherny , could you please have Claude Code build that feature?
I am confident you would love it too.
I would love to do it myself, but obviously I don't have access :)