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

Reproduce the failure by adding `argument-hint: [regression directory]` to the frontmatter in a `SKILL.md` file and loading the skill with GitHub Copilot CLI 1.0.65 on Linux x64. Trace the validation of `argument-hint`; done means this documented bracketed hint is accepted and the skill loads successfully.

Written by the indexing model from the issue text.