Contributing guide exists

Without a contributing guide, the implicit message to every potential contributor is: figure it out yourself. Developers who can't run the test suite, don't know the branch naming convention, or don't understand the PR review process submit lower-quality contributions or don't contribute at all. iso-25010:2011 analysability covers this: a project is more maintainable when its development workflow is documented rather than implicit. Generic templates ('fork the repo and submit a PR') fail entirely — they don't mention how to run the actual test suite or set up the project-specific dev environment.

Create CONTRIBUTING.md at the project root with project-specific, not generic, instructions:

# Contributing

## Development Setup

1. Fork and clone the repo
2. Install dependencies: `npm install`
3. Copy `.env.example` to `.env` and fill in values
4. Run tests: `npm test`
5. Start dev server: `npm run dev`

## Submitting Changes

- Branch from `main`
- Write tests for new behavior
- Run `npm test` before opening a PR
- PR description must explain what changed and why

Replace every step above with the commands that actually work for this project. Generic templates that don't reflect the real workflow fail this check.