fix: six methods everywhere — render_html, plugin guide, type-contracts, AGENTS

- render_html.py: 'Implement 5 methods' → '6 methods' in the protocol box and the YourPlugin tile; add missing schema() row to the protocol table; '5 methods' → '6 methods' in the explainer prose. - docs/guide/plugin-authoring-guide.md: 'these five methods' → 'six' - docs/reference/type-contracts.md: ': 5 methods' → ': 6 methods' in the type tree; 'five methods' → 'six methods' in Diagram 1 caption. - AGENTS.md: 'the five-method contract' → 'the six-method contract'

The sixth method is schema(), added when Domain Schema support was introduced. All remaining 'five' instances in the codebase refer to musical dimensions (5) or NoteKey fields (5), which are correct.