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/general-agents.ts

@@ -22,6 +22,7 @@ export function collectPendingBuiltinAgents(input: {
   uiSelectedModel?: string
   availableModels: Set<string>
   disabledSkills?: Set<string>
+  useTaskSystem?: boolean
 }): { pendingAgentConfigs: Map<string, AgentConfig>; availableAgents: AvailableAgent[] } {
   const {
     agentSources,