fix: allow variant override even with agent model config (#3163)

model-selection.ts now separates model selection from variant/reasoning
tier, so agent model overrides don't lock the variant.

34 tests pass, 5009 total, tsc clean.

Closes #3163

src/tools/delegate-task/model-selection.ts

@@ -56,6 +56,10 @@ export function resolveModelForDelegateTask(input: {
 }): { model: string; variant?: string; fallbackEntry?: FallbackEntry; matchedFallback?: boolean } | { skipped: true } | undefined {
   const userModel = normalizeModel(input.userModel)
   if (userModel) {
+    const parsed = parseUserFallbackModel(userModel)
+    if (parsed?.variant) {
+      return { model: parsed.baseModel, variant: parsed.variant }
+    }
     return { model: userModel }
   }
 
@@ -75,6 +79,10 @@ export function resolveModelForDelegateTask(input: {
       log("[resolveModelForDelegateTask] using user-configured category model (bypass validation)", {
         categoryDefaultModel: categoryDefault,
       })
+      const parsed = parseUserFallbackModel(categoryDefault)
+      if (parsed?.variant) {
+        return { model: parsed.baseModel, variant: parsed.variant }
+      }
       return { model: categoryDefault }
     }