core: allow /model switch while running; detach turnError hook context

- /model command no longer rejects switches while agent is running;
  needed for turnError hook recovery (freeloader switches model mid-error)
- TurnError hook uses context.Background()-derived context so hook can
  safely issue ctl commands without cancelling its own execution
- update TestCommand_Model_WhileRunning to expect success

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

pkg/agent/commands.go

@@ -126,10 +126,6 @@ func (s *agent) handleCommand(ctx context.Context, input string, handler EventHa
 				handler(infoEvent(s.cfg.Backend.Model()))
 				return
 			}
-			if s.IsRunning() {
-				handler(infoEvent("error: cannot switch model while agent is running"))
-				return
-			}
 			s.cfg.Backend.SetModel(args[0])
 			handler(infoEvent("switched model to: " + args[0]))
 		},

pkg/agent/core.go

@@ -310,8 +310,12 @@ func NewAgentCore(cfg AgentCoreConfig) Core {
 	a.changeCond = sync.NewCond(&a.changeMu)
 	a.pushSessionEnv()
 	a.pushLockDir()
-	a.cfg.TurnError = func(ctx context.Context, errType, errMsg string) HookResult {
-		return a.hooks.Run(ctx, HookTurnError, map[string]string{
+	a.cfg.TurnError = func(_ context.Context, errType, errMsg string) HookResult {
+		// Use a detached context so the hook can safely write "stop" to ctl
+		// without cancelling its own execution via the action context.
+		hookCtx, cancel := context.WithTimeout(context.Background(), time.Duration(hookTimeout)*time.Second)
+		defer cancel()
+		return a.hooks.Run(hookCtx, HookTurnError, map[string]string{
 			"session_id": a.sessionID,
 			"cwd":        a.CWD(),
 			"model":      a.cfg.Backend.Model(),

pkg/agent/core_test.go

@@ -1071,19 +1071,19 @@ func TestCommand_Model_WhileRunning(t *testing.T) {
 	}()
 	waitState(t, c, "thinking")
 
+	// /model is now allowed while running (needed for turnError hook recovery).
 	evs := collectEvents(context.Background(), c, "/model new-model")
 
 	close(unblock)
 	<-done
 
-	found := false
 	for _, s := range byRole(evs, "info") {
 		if strings.Contains(s, "error") {
-			found = true
+			t.Errorf("/model while running: unexpected error: %s", s)
 		}
 	}
-	if !found {
-		t.Errorf("/model while running: expected error event, got: %v", byRole(evs, "info"))
+	if got := be.Model(); got != "new-model" {
+		t.Errorf("model = %q; want new-model", got)
 	}
 }