Support for configurable models for internal sub-tasks (Example: Subagents(Explore, Task, etc) and Compaction)

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

Currently, the Copilot CLI uses the primary chat model to handle auxiliary tasks like context compaction. I propose decoupled model configuration. This would allow users to map specific sub tasks, such as summarization/compaction and task agent execution to different models based on the difficulty of the required task. For example: I would like to use a lightweight model for compaction but a more knowledgable model for task execution/exploring.

### Proposed solution

Introduce a configuration section (in config.json) that allows mapping tasks/default subagents types to specific models.

### Example prompts or workflows

_No response_

### Additional context

_No response_