fix(agents): wire useTaskSystem config flag into Sisyphus and Hephaestus

The experimental.task_system flag was defined in config but never
passed through to agent creation, so the task system prompt switch
was always off.

src/agents/builtin-agents/atlas-agent.ts

@@ -17,6 +17,7 @@ export function maybeCreateAtlasConfig(input: {
   availableSkills: AvailableSkill[]
   mergedCategories: Record<string, CategoryConfig>
   userCategories?: CategoriesConfig
+  useTaskSystem?: boolean
 }): AgentConfig | undefined {
   const {
     disabledAgents,