Add Luna.Memory with SQLite-backed persistent memory store

Implement IMemoryStore interface and MemoryStore using EF Core with
SQLite for conversation and knowledge persistence. Includes DI
service registration extensions.

Luna.Memory/Extensions/ServiceCollectionExtensions.cs

@@ -0,0 +1,14 @@
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Luna.Memory.Extensions;
+
+public static class ServiceCollectionExtensions
+{
+    extension(IServiceCollection services)
+    {
+        public IServiceCollection AddMemoryStore()
+        {
+            return services.AddSingleton<IMemoryStore, MemoryStore>();
+        }
+    }
+}