Add OG image, upgrade ontology-design skill frontmatter

imgs/og.png: standardized OG image (1280x640).
ontology-design: upgraded SKILL.md frontmatter to metadata block
format (skill_bundle, file_role, version, version_date, author).
MANIFEST frontmatter_mode corrected to claude.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: snapsynapse