Add CONTRIBUTING.md

[nora-weisser]

Goal

Add a CONTRIBUTING.md so external contributors have a single, clear entry point for setting up and contributing to the project.

Scope

Create CONTRIBUTING.md at the repo root covering:

• Getting started: fork/clone → npm install → npx playwright install → copy tests/.env.example to tests/.env.
• Branch naming: match existing style (feature/*, fix/*, test/*) and use conventional commits.
• Local checks before pushing: npm run lint, npm run format, npm run typecheck.
• How to add a test: point to tests/api/TEST_PLAN.md and the client/service/schema/fixture pattern documented in the README "API Architecture" section — link, don't duplicate.
• PR expectations: green checks, linked issue, small focused PRs.

Acceptance criteria

• CONTRIBUTING.md exists at repo root
• Links to tests/api/TEST_PLAN.md, README architecture section, and the issue tracker resolve
• Referenced npm scripts exist and work

Part of the open-source readiness epic.