Create CONTRIBUTING.md and CHANGELOG.md

[miabbott]

Add standard project documentation for contributors and release tracking.

Tasks

• Create CONTRIBUTING.md with development workflow, coding standards, and PR process
• Create CHANGELOG.md following Keep a Changelog format (https://keepachangelog.com/)
• Document how to run tests locally
• Document how to run linting

Acceptance Criteria

• CONTRIBUTING.md covers development setup and contribution process
• CHANGELOG.md documents all releases