[enhancement] Add dynamic model switching during task execution

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Background

The Copilot SDK v0.1.30 (released March 3, 2026) introduces session.setModel() for mid-session model switching, implemented by Patrick Nikoletich in commit bd98e3a.

This new API allows changing the model mid-conversation without destroying the session, preserving conversation history while taking advantage of different model capabilities.

Proposal

Implement dynamic model selection during the Execute phase to optimize cost and performance:

1. Add model configuration to Task schema - Allow users to specify which model to use per task (e.g., gpt-4.1 for complex planning, gpt-5-mini for simple file operations)

2. Implement model switching in executor.ts - Before dispatching each task, call session.setModel(task.model) to switch to the appropriate model for that specific task

3. Add UI controls in Refine screen - Allow users to assign models to tasks during the refinement phase (similar to how dependencies are edited)

4. Smart defaults - Automatically suggest models based on task complexity:

   • Complex tasks (with many dependencies, long descriptions) → gpt-4.1 or claude-sonnet-4
   • Simple tasks (single file edits, basic operations) → gpt-5-mini
   • Parallel batch optimization → use faster models for independent simple tasks

Benefit

• Cost optimization: Use cheaper/faster models for simple tasks, reserve expensive models for complex planning
• Performance improvement: Faster model = quicker execution for straightforward tasks
• Flexibility: Users can fine-tune the model selection strategy for their specific workload
• Session preservation: No need to recreate sessions when switching models, maintains conversation context

Acceptance Criteria

• Task model in src/models/plan.ts includes optional model?: string field
• src/services/executor.ts calls session.setModel() before dispatching tasks with custom models
• Task editor in src/screens/refine.tsx allows selecting model from dropdown
• Default model selection logic suggests appropriate models based on task complexity heuristics
• Session events display model_change events in the Execute screen
• Tests verify model switching behavior in src/services/executor.test.ts
• Documentation in README explains model selection strategy

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