fix(orchestrator-sisyphus): allow model override via config

Previously, orchestrator-sisyphus agent had hardcoded model and didn't
accept model parameter, making config overrides ineffective.

- Add model param to OrchestratorContext interface
- Use ctx?.model ?? DEFAULT_MODEL pattern (consistent with Sisyphus)
- Pass model override from config to createOrchestratorSisyphusAgent

src/agents/utils.ts

@@ -176,7 +176,11 @@ export function createBuiltinAgents(
 
   if (!disabledAgents.includes("orchestrator-sisyphus")) {
     const orchestratorOverride = agentOverrides["orchestrator-sisyphus"]
-    let orchestratorConfig = createOrchestratorSisyphusAgent({ availableAgents })
+    const orchestratorModel = orchestratorOverride?.model
+    let orchestratorConfig = createOrchestratorSisyphusAgent({
+      model: orchestratorModel,
+      availableAgents,
+    })
 
     if (orchestratorOverride) {
       orchestratorConfig = mergeAgentConfig(orchestratorConfig, orchestratorOverride)