docs: add CONTRIBUTING.md — guide for open source contributors

## Goal
Make it dead simple for any developer to contribute a node. The 3-file pattern is already perfect for contributors — it just needs to be documented clearly.

## Required sections
- [ ] Project overview (2-3 sentences — what NextViz is and isn't)
- [ ] Local setup instructions (`git clone`, `npm install`, `.env.nextviz.example` → `.env.nextviz`)
- [ ] How to build a node — the three-file pattern explained step by step
- `node.tsx` rules
- `panel.tsx` rules (viz-* primitives only)
- `logic.ts` rules (NodeExecutorFn signature)
- How to register in `node-executors/index.ts`
- [ ] PR checklist (no `any`, production guard, viz-* only, doc updated)
- [ ] How to run locally
- [ ] Code style notes (Tailwind, shadcn tokens, dark-first)
- [ ] Link to open `good first issue` issues

## Notes
- This file is what converts lurkers into contributors
- Keep it scannable — a new contributor should be able to ship a node in one sitting