Add CONTRIBUTING.md

[JosephDoUrden]

Summary

Add a contributor guide to help new contributors get started with the project.

What needs to be done

• Create CONTRIBUTING.md covering:
  • Fork & clone instructions
  • Branch naming convention (feat/, fix/, docs/)
  • Build & test commands
  • Code style guidelines (Biome)
  • Commit message convention (conventional commits)
  • PR submission process
  • Link to Code of Conduct

Acceptance Criteria

• Clear onboarding path for new contributors
• References existing tooling (Biome, Vitest, tsup)