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/IChatHubContext.cs

@@ -0,0 +1,13 @@
+using Luna.Shared;
+
+namespace Luna.Core.Abstractions;
+
+public interface IChatHubContext
+{
+    Task SendResponseAsync(string connectionId, ChannelMessage message, CancellationToken cancellationToken = default);
+
+    Task SendStreamingResponseAsync(
+        string connectionId,
+        IAsyncEnumerable<ChatStreamUpdate> messageStream,
+        CancellationToken cancellationToken = default);
+}

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);
+}

Luna.Core.Abstractions/Luna.Core.Abstractions.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Luna.Shared\Luna.Shared.csproj" />
+  </ItemGroup>
+
+</Project>