Render a slide from a brief via the SSG intent engine ($0.05)

Brief → SSG intent route → deterministic render. The brief path used by MCP create_slide when only a prose brief is supplied. $0.05 per slide. Unroutable briefs return a 422 intent_unroutable error with no charge. Explicit style asks in the brief (dark/light, corporate/technical/... looks) map onto built-in themes automatically unless theme_id is set; the response names the mapping and any unsupported styling.