CRITICAL: Subagents/new sessions default to "Auto" model using up GH tokens even when using BYOK

I use **BYOK** for all of my Copilot usage. However, even when running a main session with a BYOK model it insists on creating subagents/subsessions with the "Auto" model causing **significant, unintended token costs**. It basically used up my monthly Copilot token budget, and I didn't even realize it... 🫤

There needs to be a setting that controls which model is used for subagents/subsessions. Or let us completely disable GH models in the model providers. It's way too pricey anyways.

首先跟踪主会话的 BYOK 模型是如何选择的，以及 subagents 或 subsessions 如何选择 Auto 模型。确定现有的模型提供商设置及其覆盖范围；完成的标准是有一种可测试的方式来控制 subagent 模型，或在不意外使用 token 的情况下禁用 GitHub models。

由索引模型根据 Issue 内容生成。