fix(opencode): scope agent variant to model (#11410)

packages/opencode/src/config/config.ts

@@ -593,6 +593,10 @@ export namespace Config {
   export const Agent = z
     .object({
       model: z.string().optional(),
+      variant: z
+        .string()
+        .optional()
+        .describe("Default model variant for this agent (applies only when using the agent's configured model)."),
       temperature: z.number().optional(),
       top_p: z.number().optional(),
       prompt: z.string().optional(),
@@ -624,6 +628,7 @@ export namespace Config {
       const knownKeys = new Set([
         "name",
         "model",
+        "variant",
         "prompt",
         "description",
         "temperature",