feat(docs): add AI assistants guide with llms.txt documentation

- Add AI Assistants guide page with integration instructions for Cursor, GitHub Copilot, Claude, ChatGPT
- Create llms.txt (~300 lines) for concise API overview
- Create llms-full.txt (~1000 lines) for complete API documentation
- Add React Router v7 future flags to suppress deprecation warnings

Author: hyochan