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_

Start by tracing how Copilot CLI handles requests that refer to a skill by name and where its harness instructions describe skill locations. Check the .github/skills/ directory behavior and define done as skill-name requests finding the relevant skill there without an exhaustive search.

索引モデルが issue の本文から書いたものです。