Fix agent model overrides not being applied to non-factory agents

Previously, the code was explicitly removing the model property from user config overrides before merging, which prevented users from overriding agent models via config.

This change allows user config like:
{
  "agents": {
    "librarian": {
      "model": "google/gemini-3-flash-preview"
    }
  }
}

to properly override the default agent models.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/agents/utils.ts

@@ -95,8 +95,7 @@ export function createBuiltinAgents(
     }
 
     if (override) {
-      const { model: _, ...restOverride } = override
-      config = mergeAgentConfig(config, restOverride)
+      config = mergeAgentConfig(config, override)
     }
 
     result[name] = config