Use Studio CLI handoff for Figma imports

[chubes4]

Summary

• replace the hosted WordPress runner endpoint flow with a Studio CLI handoff
• export a canonical Blocks Engine site-artifact JSON payload for studio create --from
• show/copy the Studio CLI command from the Figma plugin UI
• update docs and tests around the new Studio-owned import boundary

Context

This assumes the Studio CLI import primitive from Automattic/studio#3952 lands. The Figma plugin now pressures that product path directly: the plugin prepares the source payload, while Studio owns site creation, Static Site Importer execution, and cleanup.

Testing

• npm run check --prefix plugins/figma-to-wordpress-studio
• npm test --prefix plugins/figma-to-wordpress-studio
• npm run build --prefix plugins/figma-to-wordpress-studio

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Updating the Figma plugin handoff, docs, tests, and validation.