isGptModel only matched openai/ and github-copilot/gpt- prefixes, causing
models like litellm/gpt-5.2 to fall into the Claude code path. This
injected Claude-specific thinking config, which the opencode runtime
translated into a reasoningSummary API parameter — rejected by OpenAI.
Extract model name after provider prefix and match against GPT model
name patterns (gpt-*, o1, o3, o4).
Closes#1788
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
