Add project documentation

[laurahuysamen]

Summary

• Add ARCHITECTURE.md with system boundary diagrams (Mermaid), component responsibilities, connection lifecycle, protocol layers, and key design decisions
• Add CONTRIBUTING.md with prerequisites, build/test/lint commands, code conventions, release process, and PR guidelines
• Add CLAUDE.md with AI agent guidance, key commands quick-reference, directory overview, important code patterns, and domain glossary

Test plan

• Verify all file references in the docs point to existing files
• Confirm Mermaid diagrams render correctly on GitHub
• Review content accuracy against current codebase