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.

OpenRouter/TokenTracker.cs

@@ -5,12 +5,22 @@ namespace AnchorCli.OpenRouter;
 /// </summary>
 internal sealed class TokenTracker
 {
-    private readonly ChatSession _session;
+    private ChatSession _session;
     
     public TokenTracker(ChatSession session)
     {
         _session = session;
     }
+
+    /// <summary>
+    /// Gets or sets the session. Allows setting the session after construction
+    /// to support dependency injection patterns.
+    /// </summary>
+    public ChatSession Session
+    {
+        get => _session;
+        set => _session = value;
+    }
     
     public string Provider { get; set; } = "Unknown";
     public long SessionInputTokens => _session.SessionInputTokens;