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

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

cmd/show.go

@@ -18,11 +18,12 @@ var showCmd = &cobra.Command{
 			return
 		}
 
-		if n, err := svc.GetByID(args[0]); err == nil {
-			output.PrintNode(cmd.OutOrStdout(), svc, n, jsonFlag)
-		} else {
+		n, err := svc.GetByID(args[0])
+		if err != nil {
 			fmt.Fprintln(os.Stderr, "node not found:", args[0])
+			os.Exit(1)
 		}
+		output.PrintNode(cmd.OutOrStdout(), svc, n, jsonFlag)
 	},
 }