OpenAI Agents Python 0.20.0

Implicit default model is now gpt-5.6-luna. Explicit model strings, run-level overrides and OPENAI_DEFAULT_MODEL still win. Local MCP supports Python SDK v1 and v2; apps with custom MCP HTTP auth or client factories must use the HTTP types of the installed MCP major, or pin mcp<2.⁹

Relevant to: anyone who depended on the previous default model without setting one