docs: comprehensive documentation rewrite for developers

- Rewrite CLAUDE.md with full working code examples for all patterns
  (Manager, Command, Settings, Components, State, Events, Error Handling)
- Rewrite CONTRIBUTING.md with copy-paste ready code patterns and
  step-by-step guides for adding new features
- Create ARCHITECTURE.md with system diagrams, data flow, and
  component relationships
- Create FEATURES.md documenting all core and fork-exclusive features
  including Active Listening, Ask AI, RAG, and Suggestion Engine
- Update README.md with clean overview and fork feature highlights
- Update DEV_DOCS.md with Event Catalog, Database Schema, and API Reference
- Update UPSTREAM_TRACKING.md with concise grouped PR/issue lists

Total ~3,700 lines of developer documentation covering both
frontend (React/TypeScript) and backend (Rust/Tauri) patterns.

Author: thukabjj