Switch to general LLM library from OpenAI

[korenmiklos]

BREAKING CHANGE: This is a major version update (0.4.3 -> 0.5.0)

Changes:

• Replace openai dependency with litellm for multi-provider support
• Add support for OpenAI, OpenRouter, Anthropic, and 100+ other providers
• Add api_base parameter for custom API endpoints
• Add **litellm_kwargs for provider-specific configuration
• Make api_key optional (can use environment variables)
• Update _clean_batch to use litellm.completion() with JSON mode
• Update tests to mock litellm instead of openai
• Update documentation (README.md and CLAUDE.md) with provider examples
• Update example.py with multi-provider usage examples

Benefits:

• Users can now choose from 100+ LLM providers
• Better support for OpenRouter (access multiple models through one API)
• More flexible API key management (environment variables or direct)
• Consistent interface across all providers

Migration guide:

• Existing OpenAI usage continues to work with minimal changes
• API key can now be set via OPENAI_API_KEY environment variable
• For other providers, use model prefixes (e.g., "openrouter/model-name")