bump SKILL.md version metadata to 3.20.0

Aligns with the v3.20.0 release in the private SkillCheck repo (binaries,
CHANGELOG, website banner). Content drift between private and public Free
SKILL.md tracked separately (skillcheck-ovo); this commit is a
version-only bump, no description or rules changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>