--add-dir does not load skills from added directory when running Copilot CLI outside that directory

### Describe the bug

When running Copilot CLI from a different directory and using `copilot --add-dir \add-dir-test`, Copilot CLI does not load skills that exist inside the added directory.

The issue happens even when the added directory contains valid skill definitions under all supported skill locations.

### Affected version

GitHub Copilot CLI 1.0.43.

### Steps to reproduce the behavior

1. Create the following directory structure:

```text
add-dir-test
├─.agents
│ └─skills
│ └─my-skill
│ SKILL.md
│
├─.claude
│ └─skills
│ └─my-skill
│ SKILL.md
│
├─.copilot
│ └─skills
│ └─my-skill
│ SKILL.md
│
└─.github
└─skills
└─my-skill
SKILL.md
```

2. Put the following content in each `SKILL.md`:

```markdown
---
name: my-skill
description: A brief description of what my skill does.
---

# My Skill
```

3. Open a terminal in a directory other than `add-dir-test`.
4. Run:

```powershell
copilot --add-dir \add-dir-test
```

5. Check whether Copilot CLI discovers and loads the skills inside the added directory.

### Expected behavior

Copilot CLI should discover and load skills from the directory passed to `--add-dir`, including skills located under:

- `.agents\skills\my-skill\SKILL.md`
- `.copilot\skills\my-skill\SKILL.md`
- `.github\skills\my-skill\SKILL.md`

### Additional context

- OS: Windows
- The issue occurs when Copilot CLI is started outside `add-dir-test` and the directory is added via `--add-dir`
- The skills are present and structured correctly, but they are not loaded from the added directory

首先，從 add-dir-test 以外的目錄使用 copilot --add-dir 和列出的四個 skill 位置，重現已記錄的 Windows 情況。接著追蹤 --add-dir 處理和 skill 探索的進入點，然後確認新增目錄下的有效 SKILL.md 檔案會從支援的位置載入。

由索引模型根據 Issue 內容生成。