fix: detect GPT models behind proxy providers (litellm, ollama) in isGptModel

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>

src/agents/types.ts

@@ -66,8 +66,18 @@ export interface AgentPromptMetadata {
   keyTrigger?: string
 }
 
+function extractModelName(model: string): string {
+  return model.includes("/") ? model.split("/").pop() ?? model : model
+}
+
+const GPT_MODEL_PREFIXES = ["gpt-", "gpt4", "o1", "o3", "o4"]
+
 export function isGptModel(model: string): boolean {
-  return model.startsWith("openai/") || model.startsWith("github-copilot/gpt-")
+  if (model.startsWith("openai/") || model.startsWith("github-copilot/gpt-"))
+    return true
+
+  const modelName = extractModelName(model).toLowerCase()
+  return GPT_MODEL_PREFIXES.some((prefix) => modelName.startsWith(prefix))
 }
 
 export type BuiltinAgentName =