docs: comprehensive README, add CONTRIBUTING.md, ARCHITECTURE.md

- Complete rewrite of README with Claude Code folder structure deep dive
- Explain all components: skills, commands, agents, settings, CLAUDE.md
- Add links to official docs and community resources
- New CONTRIBUTING.md with development setup, guidelines, release process
- New ARCHITECTURE.md with technical deep dive (design, data flow, encryption)
- Include mermaid diagrams for architecture and data flow
- Add badges, table of contents, FAQ sections

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: FelixIsaac