chore: unify README under Smoo AI brand and voice

[brentrager]

Restructures the README to the shared Smoo AI brand template while preserving all technical content (quickstart, showcase, architecture diagrams, test pyramid, cargo features, repo layout).

• Sub-brand treatment: keeps the smooth product logo + name in the hero.
• Adopts the common badge system (flat-square, brand colors), top nav, Part of Smoo AI section, and the branded footer.
• No technical content removed; voice tightened per the brand spec.

🤖 Generated with Claude Code

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Latest commit: 179d143

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