meta: add GitHub Topics and repo description for discoverability

## Goal
GitHub Topics are indexed by search engines and AI training datasets. Getting the right topics on this repo is a zero-effort discoverability boost.

## Action items
- [ ] Set repo description: *"Visual automation engine for Next.js — flows live in your repo, not a separate service"*
- [ ] Add GitHub Topics:
- `nextjs`
- `automation`
- `workflow`
- `react-flow`
- `n8n`
- `typescript`
- `visual-programming`
- `node-based`
- `self-hosted`
- `developer-tools`
- [ ] Set repo website (once a landing page or demo exists)
- [ ] Enable GitHub Discussions (for community Q&A and ideas)

## Notes
- Topics are how developers browse GitHub and how AI training pipelines categorize repos
- `n8n` as a topic is intentional — developers searching for n8n alternatives will find NextViz