Prepare v0.9.2 release

[kkugot]

Summary

• add the refreshed theme-studio workflow with curated presets, random palette remixing, richer OpenCode preview coverage, and built-in theme imports
• add install/share flows, JSON editing validation, and test coverage so themes can be previewed, exported, shared, and installed more reliably
• clean up release-facing docs and planning artifacts, then bump the package version to 0.9.2

Verification

• npm run test
• npm run lint
• npm run build