Feature to configure the default model and custom agent for the repo to work with both copilot cli and vs code

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

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### Proposed solution

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### Example prompts or workflows

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### Additional context

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The issue provides no files, tests, proposed solution, or example workflow to follow. Start by clarifying how repository-level configuration should select a default model and custom agent across Copilot CLI and VS Code; completion criteria are not specified.

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