test: add e2e test suite and fix namespace/mention/assignee flags

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

cmd/del.go

@@ -23,8 +23,8 @@ var delCmd = &cobra.Command{
 
 		n, err := svc.GetByID(args[0])
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "node not found: %s", args[0])
-			return
+			fmt.Fprintf(os.Stderr, "node not found: %s\n", args[0])
+			os.Exit(1)
 		}
 
 		if !dForce {