chore: glow up README — hero banner, marketing callout, diagrams

[brentrager]

What

v2 brand glow-up for the README:

• Hero banner — replaced the logo + H1 + tagline block with a gradient .github/banner.png (SVG source kept).
• Primary badges restyled to for-the-badge + labelColor=020618. TypeScript badge stays flat-square. Nav bolded.
• Marketing callout — upgraded "Part of Smoo AI" to lead with smoo.ai/open-source (plain OSS tool) + sibling packages.
• Diagrams — none in README/docs (n/a).

All technical content preserved.

🤖 Generated with Claude Code

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