feat: landing page full design — hero, features, social proof

[basanth-p]

Summary

Build out the full landing page design replacing the Sprint 1 shell in app/(marketing)/page.tsx.

Sections to build

1. Hero — headline, subheadline, GitHub CTA button, secondary OSS link
2. How it works — 3-step explainer (Connect → Score → Share)
3. Feature grid — 6 key features with icons
4. Social proof strip — contributor count, GitHub stars, early adopter logos
5. Footer — links to OSS, docs, Twitter/X, LinkedIn

Acceptance criteria

• Pixel-perfect on mobile and desktop
• CTA "Connect with GitHub" points to /connect
• "View open source" points to /oss
• Lighthouse score ≥ 90 on all metrics
• No auth overhead on this page