chore: glow up README — hero banner + marketing callout

[brentrager]

Applies the Smoo AI brand v2 glow-up to the README.

• Hero banner: gradient banner (.github/banner.png, SVG source kept) replaces the logo + H1 + tagline block. Primary 3 badges now for-the-badge + labelColor=020618; download/language badges stay flat-square; nav bolded.
• Marketing callout: "Part of Smoo AI" upgraded to lead with smoo.ai/open-source + sibling packages.
• Diagrams: none present.

All real content (utility examples, usage) preserved.

🤖 Generated with Claude Code

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Latest commit: 28f0c8f

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