Bug: /agent pop-up treats .github\agents\AGENTS.md as a custom agent

### Describe the bug

The `/agent` pop-up incorrectly attempts to load a repository guidance file at `.github\agents\AGENTS.md` as a custom agent definition. It then reports a malformed custom-agent frontmatter error, even though `AGENTS.md` is documented as repository instructions at https://agents.md/ and is not intended to be a custom agent file.

Observed output:

```text
Custom Agents

The following agents failed to load:
✖ .github\agents\AGENTS.md: custom agent markdown frontmatter is malformed: missing or malformed YAML frontmatter
```

### Affected version

Unknown; please determine from the report metadata.

### Steps to reproduce the behavior

1. Add or use a repository guidance file at `.github\agents\AGENTS.md` following the `AGENTS.md` documentation at https://agents.md/.
2. Start Copilot CLI in the repository.
3. Open the `/agent` pop-up.
4. Observe that the file is discovered as a custom agent and fails frontmatter validation.

### Expected behavior

The `/agent` pop-up should distinguish repository instruction files such as `AGENTS.md` from custom agent definitions. It should not try to parse a documented `AGENTS.md` file as a custom agent or report a frontmatter error for it.

### Additional context

- Platform: Windows
- The error is shown in the `/agent` pop-up.
- Documentation referenced: https://agents.md/
- The path is displayed with Windows separators: `.github\agents\AGENTS.md`.

Comece pelo fluxo de descoberta de agentes personalizados e validação de frontmatter do pop-up /agent, reproduzindo o relato com .github\agents\AGENTS.md no Windows. O trabalho estará concluído quando o arquivo de instruções do repositório for distinguido dos agentes personalizados, não for analisado como um deles e nenhum erro de frontmatter malformado aparecer, enquanto os agentes personalizados continuarem detectáveis.

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