Add skill: mattjoyce/okf-skill

[mattjoyce]

Adds okf to Community Skills → Context Engineering.

• Repo: https://github.com/mattjoyce/okf-skill
• What it does: Authors and validates Open Knowledge Format (OKF) bundles — vendor-neutral knowledge represented as a directory of markdown files with YAML frontmatter.
• Public repo, documented (README + SKILL.md), author prefix included, description under 10 words. Link verified.