Merge PR #3: Remove OpenRouter support, replace litellm with Chutes API

This umbrella commit combines changes from all three feature PRs:
- PR #1: Chutes API integration with Kimi K2.5-TEE model
- PR #2: Comprehensive documentation with Mermaid diagrams
- PR #3: Remove OpenRouter support, replace litellm with Chutes API

Conflicts resolved by taking the latest implementation from PR #3,
which provides a cleaner httpx-based client without litellm dependency.