Add comprehensive Configuration Guide

Create CONFIGURATION.md with complete documentation:
- All 11 LLM provider configurations with examples
- Agent behavior and model settings
- Tools configuration for security
- Chat channel integration templates
- Environment variable support
- Complete example configurations
- Troubleshooting tips

Update README.md and QUICKSTART.md to reference the new guide.
473 lines of detailed configuration documentation.

Co-authored-by: hustcc <7856674+hustcc@users.noreply.github.com>

Author: Copilot