fix(delegate-task): pass variant as top-level field in prompt body

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/tools/delegate-task/tools.ts

@@ -936,7 +936,8 @@ To continue this session: session_id="${task.sessionID}"`
                 question: false,
               },
               parts: [{ type: "text", text: args.prompt }],
-              ...(categoryModel ? { model: categoryModel } : {}),
+              ...(categoryModel ? { model: { providerID: categoryModel.providerID, modelID: categoryModel.modelID } } : {}),
+              ...(categoryModel?.variant ? { variant: categoryModel.variant } : {}),
             },
           })
         } catch (promptError) {