fix(commands): remove unintended config mutation in SwitchModel

SwitchModel should only update the routed agent's runtime Model field.
Writing to cfg.Agents.Defaults.ModelName was a behavioral change that
corrupts the default agent config when switching a non-default agent.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/agent/loop.go

@@ -1532,9 +1532,6 @@ func (al *AgentLoop) buildCommandsRuntime(agent *AgentInstance) *commands.Runtim
 		rt.SwitchModel = func(value string) (string, error) {
 			oldModel := agent.Model
 			agent.Model = value
-			if al.cfg != nil {
-				al.cfg.Agents.Defaults.ModelName = value
-			}
 			return oldModel, nil
 		}
 	}