copilot cli doesn't even know copilot skills stays at .github/skills/

### Describe the bug

A.T., every time I instruct copilot-cli change a skill by referring its name, it **doesn't know where to look**. In the end it exhausts everywhere then land in .github/skills/* by brute force search. it appears the directory convention isn't in the model's knowledge and not in the harness instructions either.

### Affected version

_No response_

### Steps to reproduce the behavior

_No response_

### Expected behavior

_No response_

### Additional context

_No response_

首先追踪 Copilot CLI 如何处理按名称引用 skill 的请求，以及其 harness 指令在哪里描述 skill 的位置。检查 .github/skills/ 目录的行为，并将完成定义为：按 skill 名称发出的请求无需进行穷尽搜索即可在那里找到相关 skill。

由索引模型根据 Issue 内容生成。