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/appsettings.json

@@ -0,0 +1,13 @@
+{
+  "Session": {
+    "ContextTokenThreshold": 0.75,
+    "RetainedMessagesAfterCompacting": 2
+  },
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.AspNetCore": "Warning"
+    }
+  },
+  "AllowedHosts": "*"
+}