docs: rewrite README — storefront overhaul for open source launch

## Goal
The README is the first thing any developer sees. Before any public launch (HN, Reddit, etc.), it needs to be compelling enough to convert a visitor into a star/contributor in under 60 seconds.

## Required sections
- [ ] One-liner value prop: *"Visual automation that lives in your Next.js repo — not a separate service"*
- [ ] 30-60 second demo GIF showing the canvas running a real flow
- [ ] `executeFlow()` code snippet showing how to call a flow from an API route
- [ ] Quick start / install instructions
- [ ] Comparison table: NextViz vs n8n (git-native, TypeScript-safe, no separate service, no lock-in)
- [ ] "Coming soon" section for `npx nextviz init` (Phase 5)
- [ ] Node roster table with status badges
- [ ] Contributing link → `CONTRIBUTING.md`

## Notes
- Lead with the problem n8n has (separate service, flows locked in DB, not in git)
- Show the solution in code first, words second
- The demo GIF is non-negotiable — it's the single highest-ROI asset for virality