feat(site): add the official Cetas website

[colmugx]

Summary

Builds the official Cetas product website, with documentation kept as a supporting surface rather than the primary experience.

Official site

• adds a warm, product-focused landing page at /
• leads with One agent. Every surface.
• includes the official Cetas pixel logo in the header, hero, and site icon
• refines the visual system with restrained 1-bit / 8-bit details: subtle grid texture, pixel-aware logo rendering, compact mono labels, stepped micro-details, and slightly retro button/card treatments
• keeps the retro direction intentionally light so typography, product UI, navigation, and documentation remain easy to read
• includes an interactive SolidJS product demo switching between Terminal, Zed / ACP, and Headless while keeping the same task visible
• explains the core product idea: different surfaces converge on the same cetas-core and Posoco agent assembly
• shows shared ~/.cetas/ state using repository-backed terminology
• adds open-source / repository presentation and clear GitHub / Get started CTAs
• adds light/dark theme support, responsive layouts, and reduced-motion handling
• adds /download and /changelog marketing routes plus favicon/robots metadata

Docs

• keeps the Starlight /docs/* documentation and navigation
• documentation remains grounded in current repository behavior

Design direction

The homepage is a developer-product site with a warm neutral base and a restrained retro-computing layer rather than a full game UI. The supplied 1-bit Cetas mark is used as the primary brand asset. Pixel references are limited to framing, typography accents, micro-decoration, and product UI details; body copy and navigation stay modern and highly legible.

Validation

• local static route/link validation passes for known routes
• local CSS structural checks pass
• GitHub Actions runs bun run check and bun run build

This change remains isolated to site/ plus the site CI workflow and does not modify Cetas implementation or release logic.