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.

UsageDisplayer.cs

@@ -0,0 +1,48 @@
+using Spectre.Console;
+
+namespace AnchorCli.OpenRouter;
+
+/// <summary>
+/// Displays token usage and cost information to the console.
+/// </summary>
+internal sealed class UsageDisplayer
+{
+    private readonly TokenTracker _tokenTracker;
+
+    public UsageDisplayer(TokenTracker tokenTracker)
+    {
+        _tokenTracker = tokenTracker;
+    }
+
+    /// <summary>
+    /// Displays the usage statistics for a single response.
+    /// </summary>
+    public void Display(int inputTokens, int outputTokens)
+    {
+        if (inputTokens > 0 || outputTokens > 0)
+        {
+            _tokenTracker.AddUsage(inputTokens, outputTokens);
+            var cost = _tokenTracker.CalculateCost(inputTokens, outputTokens);
+            var ctxPct = _tokenTracker.ContextUsagePercent;
+
+            AnsiConsole.WriteLine();
+            AnsiConsole.MarkupLine(
+                $"[dim grey]  {TokenTracker.FormatTokens(inputTokens)}↑ {TokenTracker.FormatTokens(outputTokens)}↓" +
+                $"  {TokenTracker.FormatCost(cost)}" +
+                (ctxPct >= 0 ? $"  ctx:{ctxPct:F0}%" : "") +
+                $"  │  session: {TokenTracker.FormatCost(_tokenTracker.SessionCost)}[/]");
+        }
+        else
+        {
+            AnsiConsole.WriteLine();
+        }
+    }
+
+    /// <summary>
+    /// Displays a rule separator.
+    /// </summary>
+    public void DisplaySeparator()
+    {
+        AnsiConsole.Write(new Rule().RuleStyle(Style.Parse("grey dim")));
+    }
+}