- Replace all litellm references with langchain-gigachat
- Update env vars: add GIGA_URL, GIGA_KEY, GIGA_PEM docs
- Default model: gpt-4o → GigaChat-2-Max
- Add GigaChat model table with context windows
- Update CLI options: add --giga-url, --giga-pem, --giga-key
- Update troubleshooting for GigaChat-specific errors
- Update architecture section with llm.py adapter details
- Add context.py and cost.py to project structure

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>