feat: Add SetupCommand to run an interactive setup wizard.

Commands/SetupCommand.cs

@@ -0,0 +1,13 @@
+namespace AnchorCli.Commands;
+
+public class SetupCommand : ICommand
+{
+    public string Name => "setup";
+    public string Description => "Run interactive setup wizard to configure API key and model";
+
+    public Task ExecuteAsync(string[] args, CancellationToken ct)
+    {
+        SetupTui.Run();
+        return Task.CompletedTask;
+    }
+}

Program.cs

@@ -183,6 +183,7 @@ commandRegistry.Register(new HelpCommand(commandRegistry));
 commandRegistry.Register(new ClearCommand());
 commandRegistry.Register(new StatusCommand(model, endpoint));
 commandRegistry.Register(new CompactCommand(compactor, history));
+commandRegistry.Register(new SetupCommand());
 
 var commandDispatcher = new CommandDispatcher(commandRegistry);