unexpected key in SKILL.md frontmatter

[bencium]

When loading a skill with version in SKILL.md frontmatter, Claude Code throws:

unexpected key in SKILL.md frontmatter: properties must be in ('name', 'description', 'license',
'allowed-tools', 'compatibility', 'metadata')

Reproduction:

---

name: humanizer
version: 2.2.0
description: Remove signs of AI-generated writing from text.
allowed-tools:
- Read
- Write
- Edit

Expected behavior: version should be a valid top-level frontmatter key, or the error message should
suggest using metadata as a wrapper for custom properties.

Current behavior: Skill fails to load. The only workaround is nesting version inside metadata:

---

name: humanizer
description: Remove signs of AI-generated writing from text.
allowed-tools:
- Read
- Write
- Edit
metadata:
version: 2.2.0

Suggestion: Either:

1. Add version, author, and tags as recognized top-level frontmatter keys (these are standard in
   every skill/plugin ecosystem), or
2. Silently pass unrecognized keys into metadata instead of erroring, or
3. At minimum, improve the error message to say "move unrecognized keys into metadata:"