command implementations which require async operations need to run them synchronously using .GetAwaiter().GetResult() or .Wait()

with ExecuteAsync in CommandBase,

and MainAsync in Cli.Program

it will work for both async (remove .Wait(), .GetAwaiter().GetResult()) and sync (with Task.CompletedTask or Task.FromResult).

n.b. CommandBase.Execute is difference than Task.Execute or ICommand.Execute, both of which should also provide Async variant in principle but that's a separate discussion