fix(delegate-task): build categoryModel with variant for categories without fallback chain

When a category has no CATEGORY_MODEL_REQUIREMENTS entry (e.g.
user-defined categories like solana-re), the !requirement branch
set actualModel but never built categoryModel with variant from
the user config. The bottom fallback then created categoryModel
via parseModelString alone, silently dropping the variant.

Mirror the requirement branch logic: read variant from
userCategories and resolved.config, and build categoryModel
with it.

Fixes #2538

src/tools/delegate-task/category-resolver.ts

@@ -98,6 +98,11 @@ Available categories: ${allCategoryNames}`,
       modelInfo = explicitCategoryModel || overrideModel
         ? { model: actualModel, type: "user-defined", source: "override" }
         : { model: actualModel, type: "system-default", source: "system-default" }
+      const parsedModel = parseModelString(actualModel)
+      const variantToUse = userCategories?.[args.category!]?.variant ?? resolved.config.variant
+      categoryModel = parsedModel
+        ? (variantToUse ? { ...parsedModel, variant: variantToUse } : parsedModel)
+        : undefined
     }
   } else {
     const resolution = resolveModelForDelegateTask({