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/BuiltIn/CoreTools.cs

@@ -0,0 +1,24 @@
+namespace Luna.Core.Tools.BuiltIn;
+
+public class CoreTools : IToolbox
+{
+    public string Name => "core";
+
+    public record Channel(string Name, string Health, bool IsConnected);
+    public record ListChannelsResult(IEnumerable<Channel> Channels);
+    
+    [Tool("list_channels", "List available communication channels")]
+    public ListChannelsResult ListChannels()
+    {
+        return new ListChannelsResult([
+            new Channel("cli", "healthy", true),
+            new Channel("web", "healthy", false),
+            new Channel("telegram", "healthy", false)
+        ]);
+    }
+    
+    [Tool("change_channel", "Change the active communication channel")]
+    public void ChangeChannel(string channelName)
+    {
+    }
+}