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_

首先追蹤 Copilot CLI 如何處理以名稱參照 skill 的請求，以及其 harness 指示在哪裡描述 skill 的位置。檢查 .github/skills/ 目錄的行為，並將完成定義為：依 skill 名稱提出的請求無需進行詳盡搜尋即可在該處找到相關 skill。

由索引模型根據 Issue 內容生成。