Add Luna.Core server with SignalR hub, session management, and tool system

Implement the core application server as an ASP.NET web host with:
- SignalR ChatHub for real-time client communication
- Session management with token estimation for context windowing
- Extensible tool system with attribute-based tool discovery
- Chat stream update builder for streaming AI responses
- App configuration with appsettings for development and production

Luna.Core/Services/IChatStreamUpdateBuilder.cs

@@ -0,0 +1,11 @@
+using Luna.Shared;
+using Microsoft.Extensions.AI;
+
+namespace Luna.Core.Services;
+
+public interface IChatStreamUpdateBuilder
+{
+    ChatStreamUpdate FromAiContent(AIContent content, string messageId);
+    ChatStreamUpdate FromString(string content, string messageId);
+    ChatStreamUpdate CreateCompleteUpdate(string messageId);
+}