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/Tools/ToolAttribute.cs

@@ -0,0 +1,8 @@
+namespace Luna.Core.Tools;
+
+[AttributeUsage(AttributeTargets.Method)]
+public class ToolAttribute(string name, string description) : Attribute
+{
+    public string Name { get; } = name;
+    public string Description { get; } = description;
+}