docs: Add comprehensive contribution guidelines

[chrobson]

Summary

• Add detailed CONTRIBUTING.md with comprehensive guidelines for contributors
• Covers development setup, code standards, testing patterns, and documentation requirements
• Tailored to the project's unique architecture and self-testing patterns

Changes

• CONTRIBUTING.md: New comprehensive contribution guide including:
  • Development setup and prerequisites (Go 1.24+, zero dependencies)
  • Code standards (4-space indentation, 80-char line length, SPDX headers)
  • Project structure explanation (pkg/ and internal/ organization)
  • Testing guidelines with self-testing patterns using tester package
  • Documentation standards for packages and inline code
  • Pull request process and commit message format
  • Release management workflow
  • Design principles (zero dependencies, modular design, fluent APIs)

Test plan

• Verify file formatting and structure
• Check all links and references are accurate
• Review code examples for correctness
• Ensure guidelines match existing project patterns

The guidelines are now ready for review and can be proposed to the project maintainers.