[FEATURE] Add planModeModel setting to use different model for planning vs implementation

Preflight Checklist

• [ ] 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

When working on complex tasks, the optimal workflow is to use Opus for planning (better reasoning, architecture decisions) and Sonnet for implementation (faster, cost-effective for straightforward coding).

Currently, I have to manually switch models with Alt+P every time I enter/exit plan mode. This is tedious and easy to forget.

Proposed Solution

Add a planModeModel setting in settings.json:

{
  "model": "sonnet",
  "planModeModel": "opus"
}

When entering plan mode (Shift+Tab), Claude would automatically use the planModeModel. When exiting plan mode and implementing, it would switch back to the default model.

Alternative Solutions

• Manually switching models with Alt+P - works but tedious and easy to forget
• Using Opus for everything - expensive and slower for simple implementation tasks
• Using Sonnet for everything - misses out on Opus's superior planning capabilities

Priority

Medium - Quality of life improvement

Feature Category

Configuration and settings

Use Case Example

1. User enters plan mode with Shift+Tab
2. Claude automatically switches to Opus for better architectural reasoning
3. User approves the plan and exits plan mode
4. Claude automatically switches back to Sonnet for implementation
5. User saves money and time without manual intervention

Additional Context

This would optimize both cost and quality:

• Opus excels at architectural decisions and planning
• Sonnet is ideal for straightforward implementation
• Automatic switching removes friction from the optimal workflow