[P1] Create CONTRIBUTING.md with contributor guidelines

[sfloess]

Problem

Project lacks a CONTRIBUTING.md file, which is a standard for professional open-source projects.

Impact

• Severity: MEDIUM
• Category: Professional Standards
• Unclear how external contributors should:
  • Submit pull requests
  • Report bugs
  • Follow code style
  • Run tests
• May deter potential contributors

Required Content

1. How to file issues - bug reports vs feature requests
2. PR submission process - fork, branch naming, PR description
3. Code style requirements - checkstyle, formatting
4. Testing requirements - unit tests required for all changes
5. Build instructions - how to build and run tests
6. CLA/DCO requirements (if any)
7. Code of Conduct reference
8. Contact information - how to ask questions

Example Template

See https://github.com/github/docs/blob/main/CONTRIBUTING.md

Priority

P1 - High - Should be created before v1.2 release.

Related

• CODE_OF_CONDUCT.md (already exists ✅)
• README.md (has build instructions)
• .github/workflows/quality-gate.yml (defines quality requirements)