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.

Start by tracing the CLI's skill discovery and available-skill registry, comparing the supported ~/.copilot/skills path with ~/.agents/skills on Windows. Reproduce the issue with the minimal test-skill described in the report, then verify that skills under ~/.agents/skills are discovered and invoked or that the CLI clearly reports the directory as unsupported.

Written by the indexing model from the issue text.