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/plugin-handlers/agent-config-handler.ts

@@ -58,6 +58,7 @@ export async function applyAgentConfig(params: {
     params.pluginConfig.browser_automation_engine?.provider ?? "playwright";
   const currentModel = params.config.model as string | undefined;
   const disabledSkills = new Set<string>(params.pluginConfig.disabled_skills ?? []);
+  const useTaskSystem = params.pluginConfig.experimental?.task_system ?? false;
 
   const builtinAgents = await createBuiltinAgents(
     migratedDisabledAgents,
@@ -71,6 +72,7 @@ export async function applyAgentConfig(params: {
     browserProvider,
     currentModel,
     disabledSkills,
+    useTaskSystem,
   );
 
   const includeClaudeAgents = params.pluginConfig.claude_code?.agents ?? true;
@@ -101,7 +103,6 @@ export async function applyAgentConfig(params: {
       sisyphus: builtinAgents.sisyphus,
     };
 
-    const useTaskSystem = params.pluginConfig.experimental?.task_system ?? false;
     agentConfig["sisyphus-junior"] = createSisyphusJuniorAgentWithOverrides(
       params.pluginConfig.agents?.["sisyphus-junior"],
       undefined,