refactor: apply Single Responsibility Principle to Program.cs and ReplLoop.cs

extracted responsibilities from Program.cs (208→46 lines) and ReplLoop.cs (274→174 lines) into focused service classes: HeaderRenderer, SessionManager, ApplicationStartup, ResponseStreamer, SpinnerService, UsageDisplayer, and ContextCompactionService. Each class now has a single, well-defined responsibility, improving testability and maintainability.

ResponseStreamer.cs

@@ -0,0 +1,60 @@
+using Microsoft.Extensions.AI;
+using OpenAI;
+
+namespace AnchorCli;
+
+/// <summary>
+/// Handles streaming responses from the chat client, including token usage capture.
+/// </summary>
+internal sealed class ResponseStreamer
+{
+    private readonly ChatSession _session;
+
+    public ResponseStreamer(ChatSession session)
+    {
+        _session = session;
+    }
+
+    /// <summary>
+    /// Streams a response from the session and captures token usage.
+    /// Returns an async enumerable that yields text chunks as they arrive.
+    /// </summary>
+    public async IAsyncEnumerable<string> StreamAsync(
+        [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken)
+    {
+        await using var stream = _session
+            .GetStreamingResponseAsync(cancellationToken)
+            .GetAsyncEnumerator(cancellationToken);
+
+        int respIn = 0, respOut = 0;
+
+        void CaptureUsage(ChatResponseUpdate update)
+        {
+            if (update.RawRepresentation is OpenAI.Chat.StreamingChatCompletionUpdate raw
+                && raw.Usage != null)
+            {
+                respIn = raw.Usage.InputTokenCount;
+                respOut += raw.Usage.OutputTokenCount;
+            }
+        }
+
+        // Stream all chunks
+        while (await stream.MoveNextAsync())
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            CaptureUsage(stream.Current);
+            var text = stream.Current.Text;
+            if (!string.IsNullOrEmpty(text))
+            {
+                yield return text;
+            }
+        }
+
+        // Store final usage stats
+        LastInputTokens = respIn;
+        LastOutputTokens = respOut;
+    }
+
+    public int LastInputTokens { get; private set; }
+    public int LastOutputTokens { get; private set; }
+}