argument-hint format validation issue

### Describe the bug

The skills now failed to load with:
✖ .../SKILL.md: argument-hint must be a string

This is my setting:
argument-hint: [regression directory]

And from both vscode and claude spec:
https://code.visualstudio.com/docs/agent-customization/agent-skills

argument-hint | No | Hint text shown in the chat input field when the skill is invoked as a slash command. Helps users understand what additional information to provide (for example, [test file] [options]).
-- | -- | --

https://code.claude.com/docs/en/skills

argument-hint | No | Hint shown during autocomplete to indicate expected arguments. Example: [issue-number] or [filename] [format].
-- | -- | --

### Affected version

GitHub Copilot CLI 1.0.65.

### Steps to reproduce the behavior

Add this to SKILL.md frontmatter:
argument-hint: [regression directory]

### Expected behavior

Success to load the skill

### Additional context

Linux x64

通过将 `argument-hint: [regression directory]` 添加到 `SKILL.md` 文件的 frontmatter 中，并在 Linux x64 上使用 GitHub Copilot CLI 1.0.65 加载 skill，重现该失败。跟踪 `argument-hint` 的验证过程；当这个文档化的方括号提示被接受且 skill 成功加载时，即表示完成。

由索引模型根据 Issue 内容生成。