fix: only call setDefaultAgentForSort when user explicitly sets default_agent

The previous code read params.config.default_agent which is always populated to
"Sisyphus - Ultraworker" by existing logic (lines 196-204) even when the user
never configured default_agent. This silently overrode any custom agent_order.

Now we gate on configuredDefaultAgent (the user's explicit value) so the sort
shim rank map is only mutated when the user actually set default_agent.

Add regression test: agent_order without default_agent must preserve order.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

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

@@ -400,9 +400,10 @@ export async function applyAgentConfig(params: {
     );
   }
 
-  const resolvedDefault = (params.config as { default_agent?: string }).default_agent;
-  if (resolvedDefault) {
-    setDefaultAgentForSort(resolvedDefault);
+  if (configuredDefaultAgent) {
+    setDefaultAgentForSort(
+      (params.config as { default_agent?: string }).default_agent ?? configuredDefaultAgent,
+    );
   }
 
   const agentResult = params.config.agent as Record<string, unknown>;