Add agent system with Core and Librarian agent implementations

Define IAgent interface and implement CoreAgent (main conversational
agent) and LibrarianAgent (knowledge retrieval agent). Includes DI
service collection extensions for agent registration.

Luna.Agents.Abstractions/IAgent.cs

@@ -0,0 +1,19 @@
+using Luna.Configuration;
+using Microsoft.Extensions.AI;
+
+namespace Luna.Agents.Abstractions;
+
+public interface IAgent
+{
+    public static virtual string Name { get; } = null!;
+    
+    public string DisplayName { get; }
+    public string Description { get; }
+    public AgentOptions Options { get; init; }
+
+    Task<IEnumerable<ChatMessage>> ProcessAsync(IReadOnlyList<ChatMessage> messages,
+        CancellationToken ct = default);
+    
+    IAsyncEnumerable<ChatResponseUpdate> ProcessStreamingAsync(IReadOnlyList<ChatMessage> messages,
+        CancellationToken ct = default);
+}