Mark CommandManager as obsolete; add warning suppression

Updated `CommandManager` to mark the `_envelopeReceiverService` field and its constructor as obsolete, recommending the use of `MediatR`. Added a new obsolete property `EnvelopeReceiver` for accessing the service. In `Program.cs`, added warning suppression for the obsolete member during command manager runner registration.

EnvelopeGenerator.Terminal/Program.cs

@@ -15,7 +15,9 @@ public class Program
 
         var config = builder.Configuration;
 
+#pragma warning disable CS0618 // Type or member is obsolete
         builder.Services.AddCommandManagerRunner(config);
+#pragma warning restore CS0618 // Type or member is obsolete
 
         var app = builder.Build();