docs: Restructure documentation with Bookstack-compatible hierarchy

- Organized docs into numbered shelf/book/chapter/page structure
- Created comprehensive README.md with project overview
- Added Getting Started book (quick start, installation, configuration)
- Added Development book (workflow, testing, type sharing)
- Added Architecture book (design principles, PDA-friendly patterns)
- Added API Reference book (conventions, authentication)
- Moved TYPE-SHARING.md to proper location
- Updated all cross-references in main README
- Created docs/README.md as master index
- Removed old QA automation reports
- Removed deprecated SETUP.md (content split into new structure)

Documentation structure follows Bookstack best practices:
- Numbered books: 1-getting-started, 2-development, 3-architecture, 4-api
- Numbered chapters and pages for ordering
- Clear hierarchy and navigation
- Cross-referenced throughout

Complete documentation available at: docs/README.md

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

docs/3-architecture/README.md

@@ -0,0 +1,29 @@
+# Architecture
+
+Technical architecture and design principles for Mosaic Stack.
+
+## Chapters
+
+1. **Overview** — System design and component architecture
+2. **Authentication** — BetterAuth integration and OIDC flow
+3. **Design Principles** — PDA-friendly patterns and UX guidelines
+
+## Architecture Principles
+
+- **Type Safety First:** Shared types prevent frontend/backend drift
+- **Multi-Tenant Ready:** Row-level security with workspace isolation
+- **PDA-Friendly:** Calm, stress-free language and visual design
+- **Modular:** Plugin architecture for extensibility
+
+## Technology Decisions
+
+Key architectural choices and their rationale:
+- **BetterAuth** over Passport.js for modern authentication
+- **Prisma ORM** for type-safe database access
+- **Monorepo** with pnpm workspaces for code sharing
+- **PostgreSQL + pgvector** for AI-ready data storage
+
+## Next Steps
+
+- Review **API** for implementation details
+- Check **Development** for workflow standards