[FEATURE] Support argument-hint frontmatter in SKILL.md files

Summary

After the commands/skills merge (#13115), the argument-hint frontmatter field works in .claude/commands/*.md files but doesn't appear in autocomplete for skills defined in .claude/skills/*/SKILL.md.

Since the merge unified commands and skills, skills should support the same frontmatter fields - including argument-hint for autocomplete hints.

Current Behavior

When I type /brain in the CLI, no argument hints are shown, even with this frontmatter:

---
name: brain
description: "Collaborative scratchpad for planning and research..."
argument-hint: "[{topic} | plan|research|review {topic} | idea|add {text} | check|done]"
allowed-tools: [Read, Write, Glob, Grep, Bash]
---

Expected Behavior

Typing /brain should show the argument hint in autocomplete: [{topic} | plan|research|review {topic} | idea|add {text} | check|done]

This worked when we had separate command files in .claude/commands/brain.md.

Context

• Commands and skills were merged in #13115 to "simplify the mental model"
• argument-hint was a valuable feature for discoverability - users could see available subcommands at a glance
• The slash commands documentation shows argument-hint as a supported field, but it's unclear if this applies to SKILL.md files

Suggested Fix

Parse and display argument-hint from SKILL.md frontmatter the same way it's handled for command files.

Environment

• Claude Code version: 2.1.x
• Platform: macOS