[BUG] 'minimal' value is not support with the 'gpt-5.1' model

[WAY29]

{
    "error": {
        "message": "Unsupported value: 'minimal' is not supported with the 'gpt-5.1' model. Supported values are: 'none', 'low', 'medium', and 'high'.",
        "type": "invalid_request_error",
        "param": "***.effort",
        "code": "unsupported_value"
    }
}

https://github.com/sst/opencode/blob/ce6436280af6aa1bb5aef6c711a185956cdc56bd/packages/opencode/src/session/prompt.ts#L1398-L1400

Maybe set to low/none if using gpt-5.1 or gpt-5.1-codex, Reference: https://platform.openai.com/docs/api-reference/assistants/createAssistant#assistants_createassistant-reasoning_effort