refactor: rename Kopaya to Eryx

Rename the product, runtime surfaces, sidecar projects, docs, tests, and packaging outputs from Kopaya to Eryx across the repository.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

sidecar/src/Eryx.AgentHost/Program.cs

@@ -0,0 +1,10 @@
+using Eryx.AgentHost.Services;
+
+if (!args.Contains("--stdio", StringComparer.Ordinal))
+{
+    Console.Error.WriteLine("Eryx.AgentHost expects the --stdio flag.");
+    return;
+}
+
+SidecarProtocolHost host = new();
+await host.RunAsync(Console.In, Console.Out, CancellationToken.None);