Allow custom Auto model-routing preferences

## Feature request

Allow users to configure task-aware preferences for **Auto** model selection in the GitHub Copilot app.

For example:

- Prefer GPT-5.6 Terra for straightforward coding tasks, focused edits, simple questions, and routine explanations.
- Prefer GPT-5.6 Sol for architecture/design work, complex debugging, security review, multi-file refactors, and other involved changes.

These preferences should be advisory rather than absolute: the router may still choose another enabled model when availability, reliability, cost, or enterprise policy requires it.

Custom instructions currently affect response behavior but do not influence the model selected by Auto. A dedicated setting would make Auto routing more predictable while preserving enterprise model-access controls.

该 issue 未指明任何文件、测试或入口点。首先定位应用中的 Auto 模型选择设置和路由行为，然后定义建议性偏好如何与可用性、可靠性、成本和企业策略交互；完成的标准是用户可以配置面向任务的偏好，同时不绕过模型访问控制。

由索引模型根据 Issue 内容生成。