fix: model fallback properly falls through to system default

- Remove Step 3 in model-resolver that forced first fallbackChain entry
  even when unavailable, blocking system default fallback
- Add sisyphusJuniorModel option to delegate_task so agents["Sisyphus-Junior"]
  model override is respected in category-based delegation
- Update tests to reflect new fallback behavior

src/index.ts

@@ -236,6 +236,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
     directory: ctx.directory,
     userCategories: pluginConfig.categories,
     gitMasterConfig: pluginConfig.git_master,
+    sisyphusJuniorModel: pluginConfig.agents?.["Sisyphus-Junior"]?.model,
   });
   const disabledSkills = new Set(pluginConfig.disabled_skills ?? []);
   const systemMcpNames = getSystemMcpServerNames();