Add project documentation (ARCHITECTURE, CONTRIBUTING, CLAUDE)

[laurahuysamen]

Summary

• Add ARCHITECTURE.md with system context diagram, connection lifecycle sequence diagram, component breakdown, dependency table, and architectural principles
• Add CONTRIBUTING.md with prerequisites, build/test commands, code style guidelines, branch naming conventions, PR process, and release workflow
• Add CLAUDE.md with quick-reference commands, directory structure overview, key patterns/conventions, domain terminology glossary, and common pitfalls — cross-references the other two docs to avoid duplication

Test plan

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