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/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
+);