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

[brentrager]

README glow-up (v2)

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

• Hero banner — gradient .github/banner.png (2× rasterized from .github/banner.svg) replaces the plain logo header.
• Primary badges restyled to for-the-badge + labelColor=020618; secondary/language badges stay flat-square. Bold nav row.
• Marketing callout in the "Part of Smoo AI" section linking the most relevant smoo.ai destination + sibling repos.

All real technical content preserved. Docs-only change.

🤖 Generated with Claude Code

Diagrams: 3 mermaid blocks simplified + brand-themed (dark palette init directive + warm/teal accent classDefs); verified they parse with mermaid 11.

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Latest commit: 91b6535

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