docs: add CONTRIBUTING.md

## Summary This repo has no `CONTRIBUTING.md`, unlike the contract repo (NovaEvents) which documents setup, testing, and PR conventions clearly. ## Context The project actively receives outside PRs on this repo, but there's nothing telling a new contributor how to run `npm run lint`, `npm test`, or `npm run build` locally, or what's expected before opening a PR. ## Acceptance Criteria - [ ] `CONTRIBUTING.md` covering: prerequisites, `npm install`, `npm run dev`, `npm run lint`, `npm test`, `npm run build` - [ ] Document that lint, tests, and build must all pass before opening a PR - [ ] Link to it from the README