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/Hubs/ChatHubContext.cs

@@ -0,0 +1,30 @@
+using Luna.Core.Abstractions;
+using Luna.Shared;
+using Microsoft.AspNetCore.SignalR;
+
+namespace Luna.Core.Hubs;
+
+public class ChatHubContext(IHubContext<ChatHub> hubContext) : IChatHubContext
+{
+    public async Task SendResponseAsync(string connectionId, ChannelMessage message, CancellationToken cancellationToken = default)
+    {
+        await GetClient(connectionId)
+            .SendAsync("OnMessageReceived", message, cancellationToken);
+    }
+
+    public async Task SendStreamingResponseAsync(
+        string connectionId,
+        IAsyncEnumerable<ChatStreamUpdate> messageStream,
+        CancellationToken cancellationToken = default)
+    {
+        var client = GetClient(connectionId);
+
+        await foreach (var update in messageStream.WithCancellation(cancellationToken))
+        {
+            await client.SendAsync("OnMessageStreamReceived", update, cancellationToken);
+        }
+    }
+
+    private ISingleClientProxy GetClient(string connectionId)
+        => hubContext.Clients.Client(connectionId);
+}