**Please add a persistent default agent setting.

**Please add a persistent default agent setting.** Agent choice resets to base Copilot every new chat, so I must re-pick my custom agent each time. The app already remembers my default **model** — please do the same for **agents** (Settings dropdown or a `defaultAgent` key in `settings.json`).

---
| Field | Value |
| --- | --- |
| App version | 1.0.21 |
| OS | Windows 10.0.26200 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Day 4 (Week 1) |

从 /chat 流程和 Settings 下拉菜单开始，然后跟踪应用如何持久化现有的默认模型。检查 settings.json 是否是相关的配置位置。当开始新聊天时，选定的自定义代理仍然是默认代理，并且保留现有的模型设置行为，即表示完成。

由索引模型根据 Issue 内容生成。