Rewrite design track D1: Design brief (master prompt) #338

Part of #326.

I write docs/rewrite/design-brief.md, the master prompt for the designer:

• Product philosophy: an OS with a filesystem and applications
• Surfaces: Studio desktop, admin board, phone, home site
• Requirements: high contrast, Light and Dark themes, one token source for web and phone, strict layering (tokens → atoms → composites → OS chrome) with no page-specific pieces in the package, desktop density, motion, accessibility
• Deliverables: tokens, atoms, window/taskbar/desktop/file-tile chrome, an own icon language with separate app icons and UI glyphs, marks per client with the environment colours for production, staging and development
• Hand-off format, and what to keep from today