docs: Add comprehensive documentation with Mermaid diagrams

- Create docs/ directory with 11 structured Markdown files
- Add Mermaid diagrams for architecture, agent loop, context management
- Document Chutes API integration (CHUTES_API_TOKEN, Kimi K2.5-TEE)
- Include usage examples with python3 agent.py --instruction
- Cover configuration, tools reference, best practices
- Update README.md to reference new documentation structure