Add Luna.Shared library with shared DTOs and models

Introduce ChannelMessage and ChatStreamUpdate types used across the
solution for inter-project communication between channels, core, and
the web interface.

Luna.Shared/ChannelMessage.cs

@@ -0,0 +1,10 @@
+namespace Luna.Shared;
+
+public record ChannelMessage(
+    string MessageId,
+    string ConversationId,
+    string SenderId,
+    string SenderName,
+    string Content,
+    DateTimeOffset Timestamp,
+    Dictionary<string, string>? Metadata = null);

Luna.Shared/ChatStreamUpdate.cs

@@ -0,0 +1,19 @@
+namespace Luna.Shared;
+
+public enum ChatStreamUpdateType
+{
+    Thinking,
+    Text,
+    ToolCall,
+    ToolResult,
+    Usage,
+    Complete
+}
+
+public record ChatStreamUpdate(
+    ChatStreamUpdateType Type,
+    string? Content,
+    string? Role,
+    bool IsComplete,
+    string? MessageId = null
+);

Luna.Shared/Luna.Shared.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <AllowMissingPrunePackageData>true</AllowMissingPrunePackageData>
+  </PropertyGroup>
+
+</Project>