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

cmd/update.go

@@ -55,11 +55,11 @@ var updateCmd = &cobra.Command{
 			ok, blockers, err := svc.CanClose(args[0])
 			if err != nil {
 				fmt.Fprintln(os.Stderr, "failed to check blockers:", err)
-				return
+				os.Exit(1)
 			}
 			if !ok {
 				fmt.Fprintf(os.Stderr, "cannot close: blocked by %v\n", blockers)
-				return
+				os.Exit(1)
 			}
 		}