[Help Wanted]: How do I use custom commands?

### Before Reporting

- [x] I have checked FAQ, and there is no solution to my issue
- [x] I have searched the existing issues, and there is no existing issue for my issue

### Describe your issue

How do I use custom commands? When I type '/' in the chat, I can only find 'shell' and 'shell shortcut'.

先查看 FAQ 和现有的 issue 讨论，然后检查 CopilotForXcode 中如何记录自定义命令。完成的标准是说明如何使用自定义命令，以及为什么聊天目前只列出“shell”和“shell shortcut”。

由索引模型根据 Issue 内容生成。