Add three detailed platform guides:
1. **MULTI_LLM_SUPPORT.md** - Complete multi-platform overview
- Supported platforms comparison table
- Quick start for all platforms
- Installation options
- Complete workflow examples
- Advanced usage and troubleshooting
- Programmatic API usage examples
2. **GEMINI_INTEGRATION.md** - Google Gemini integration guide
- Setup and API key configuration
- Complete workflow with tar.gz packaging
- Gemini-specific format differences
- Files API + grounding usage
- Cost estimation and best practices
- Troubleshooting common issues
3. **OPENAI_INTEGRATION.md** - OpenAI ChatGPT integration guide
- Setup and API key configuration
- Complete workflow with Assistants API
- Vector Store + file_search integration
- Assistant instructions format
- Cost estimation and best practices
- Troubleshooting common issues
All guides include:
- Code examples for CLI and Python API
- Platform-specific features and differences
- Real-world usage patterns
- Troubleshooting sections
- Best practices
Related to #179
