[BUG] argument-hint frontmatter not displayed for plugin skills in slash command autocomplete

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

argument-hint in SKILL.md frontmatter is not displayed in the slash command autocomplete UI for plugin skills, while it works correctly for project-level skills (.claude/skills/).

Both use the exact same SKILL.md format and frontmatter structure — the only difference is where the file is located.

Steps to Reproduce

1. Create a plugin with a skill that has argument-hint in frontmatter:

---
name: my-skill
description: "My skill description"
argument-hint: "<arg1> [--option]"
---

1. Load the plugin via --plugin-dir ./plugins/my-plugin or install from a marketplace
2. Type /my-skill in the Claude Code prompt
3. Observe that the argument hint is not displayed in the autocomplete dropdown

1. Now create a project-level skill at .claude/skills/my-skill/SKILL.md with the exact same frontmatter
2. Type /my-skill in the Claude Code prompt
3. Observe that the argument hint is displayed correctly

Expected Behavior

argument-hint should be displayed in the slash command autocomplete UI for both plugin skills and project-level skills, since both use the same SKILL.md format.

Actual Behavior

| Skill location | argument-hint displayed |
|---|---|
| .claude/skills/ (project-level) | ✅ Yes |
| plugins/*/skills/ (plugin, via --plugin-dir) | ❌ No |
| plugins/*/skills/ (plugin, via marketplace) | ❌ No |

Environment

• Claude Code version: v2.1.101
• OS: macOS (Darwin 23.6.0)
• Shell: zsh