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

`SKILL.md` ファイルの frontmatter に `argument-hint: [regression directory]` を追加し、Linux x64 上で GitHub Copilot CLI 1.0.65 を使って skill を読み込むことで、失敗を再現します。`argument-hint` の検証を追跡します。文書化されたこの角括弧付きのヒントが受け入れられ、skill が正常に読み込まれれば完了です。

索引モデルが issue の本文から書いたものです。