refactor(ai): remove hardcoded gpt-4o fallback model

Remove the hardcoded "gpt-4o" fallback in cmd_ai_start to ensure
model selection relies exclusively on provider configuration defaults.

src/command/cmd_funcs.c

@@ -10947,7 +10947,7 @@ cmd_ai_start(ProfWin* window, const char* const command, gchar** args)
 
     if (!model) {
         cons_show_error("No model specified and no default model set for provider '%s'.", provider_name);
-        cons_show("Use '/ai set default-model %s <model>' or '/ai switch %s <model>'.", provider_name, provider_name);
+        cons_show("Use '/ai set default-model %s <model>'.", provider_name);
         return TRUE;
     }