Regression: skill tool cannot find valid skills in ~/.agents/skills

## Description

The `skill` tool cannot find or invoke a valid skill installed under `~/.agents/skills`, even though the skill directory and `SKILL.md` are present.

This appears to be a regression or an incomplete fix related to #2230, which is closed.

## Environment

- Copilot CLI: `1.0.78`
- OS: Windows
- Skill path: `C:\Users\omzig\.agents\skills\setup-matt-pocock-skills\SKILL.md`

## Reproduction

1. Create or install a skill at `~/.agents/skills/setup-matt-pocock-skills/SKILL.md`.
2. Ensure its frontmatter includes:

```yaml
---
name: setup-matt-pocock-skills
description: Configure this repo for the engineering skills.
disable-model-invocation: true
---
```

3. Invoke the skill tool with `setup-matt-pocock-skills`.

## Actual result

The tool returns:

```text
Skill not found: setup-matt-pocock-skills
```

The skill is also absent from the tool's available-skill registry, despite the file being present and having valid frontmatter.

## Expected result

Skills in `~/.agents/skills` should either be discovered by the CLI automatically or produce a clear configuration/status indication explaining that the directory is unsupported.

## Workaround

Copying or linking the skill into a directory that the CLI does discover appears to be required.

## Additional validation: minimal test skill

To rule out a problem specific to the original skill, I created a minimal skill with valid frontmatter at:

`C:\Users\omzig\.agents\skills\test-skill\SKILL.md`

Its contents were:

```markdown
---
name: test-skill
description: A simple test skill that confirms whether skills installed in ~/.agents/skills are discovered and invoked.
---

# Test Skill

When this skill is invoked, respond with exactly:

`test-skill loaded successfully`
```

Invoking `skill(test-skill)` produced the same result:

```text
Skill not found: test-skill
```

This validates that the failure is with out-of-box discovery of `~/.agents/skills`, not with the original skill's complexity or frontmatter.

## Follow-up validation: supported path works

I moved the same minimal `test-skill` directory from:

`C:\Users\omzig\.agents\skills\test-skill`

to:

`C:\Users\omzig\.copilot\skills\test-skill`

After moving it, invoking `skill(test-skill)` succeeded and the skill returned:

```text
test-skill loaded successfully
```

This confirms that the skill content and frontmatter are valid. The failure is specifically that `~/.agents/skills` is not included in the CLI's out-of-box discovery paths, while `~/.copilot/skills` is.

Bắt đầu bằng cách lần theo cơ chế phát hiện skill của CLI và registry các skill khả dụng, đồng thời so sánh đường dẫn được hỗ trợ ~/.copilot/skills với ~/.agents/skills trên Windows. Tái hiện vấn đề bằng test-skill tối thiểu được mô tả trong báo cáo, sau đó xác minh rằng các skill trong ~/.agents/skills được phát hiện và gọi, hoặc CLI thông báo rõ ràng rằng thư mục này không được hỗ trợ.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.