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:

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.

从 /agent 弹出窗口的自定义代理发现和 frontmatter 验证流程入手，使用 Windows 上的 .github\agents\AGENTS.md 重现该报告。完成的标准是：仓库指令文件能够与自定义代理区分开，不会被解析为自定义代理，并且不会出现 frontmatter 格式错误，同时自定义代理仍然可以被发现。