Add CONTRIBUTING.md for local dev, tests, and lint

[blinkerbit]

Source: gitlab-issues-export.md (Issue 1)

Weight: 1

Problem / context

New contributors must discover pytest, npm run lint:js, Sonar scope (sonar-project.properties), and UI guidelines (docs/developer-ui-quality.md) by exploration. codereview.md recommends CI enforcement but onboarding steps are not centralized.

Suggested approach / acceptance criteria

• Add CONTRIBUTING.md at repo root: Python version, venv, pytest, optional npm install, npm run lint:js.
• Link to docs/developer-ui-quality.md and note primary package path aird/.
• Short before-PR checklist (tests + lint).