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.Abstractions/ISessionManager.cs

@@ -0,0 +1,12 @@
+using Luna.Shared;
+
+namespace Luna.Core.Abstractions;
+
+public interface ISessionManager
+{
+    IAsyncEnumerable<ChatStreamUpdate> RouteMessagesAsync(string content,
+        string conversationId,
+        string connectionId,
+        CancellationToken ct = default);
+    Task ClientDisconnectedAsync(string connectionId);
+}