Add comprehensive development planning documents

- DEVELOPMENT_ROADMAP.md: Complete 8-phase development plan covering all future possibilities
- PRIORITIES.md: Quick reference for immediate priorities and sprint planning
- ARCHITECTURE.md: Visual diagrams and architectural overview
- Update README.md with links to planning documents and improved contribution guide

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot