feat(KB-113): fix spec editor layout in task detail modal

- Add CSS for spec tab layout with proper overflow handling
- Update TaskDetailModal spec tab wrapper class for correct styling
- Add changeset for the layout fix
- Remove stale executor tests and unused transition logic

packages/dashboard/app/components/TaskDetailModal.tsx

@@ -476,7 +476,7 @@ export function TaskDetailModal({
               onClose={() => setActiveTab("definition")}
             />
           ) : activeTab === "spec" ? (
-            <div className="detail-section">
+            <div className="detail-section detail-section--spec">
               <SpecEditor
                 content={task.prompt || ""}
                 onSave={handleSaveSpec}