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.

SessionManager.cs

@@ -0,0 +1,83 @@
+using Spectre.Console;
+
+namespace AnchorCli;
+
+/// <summary>
+/// Manages session persistence, including auto-load on startup and auto-save on exit.
+/// </summary>
+internal sealed class SessionManager
+{
+    private readonly ChatSession _session;
+    private readonly string _sessionPath;
+
+    public SessionManager(ChatSession session, string sessionPath = ".anchor/session.json")
+    {
+        _session = session;
+        _sessionPath = sessionPath;
+    }
+
+    /// <summary>
+    /// Attempts to load a session from disk. Returns true if successful.
+    /// </summary>
+    public async Task<bool> TryLoadAsync(CancellationToken cancellationToken = default)
+    {
+        if (!File.Exists(_sessionPath))
+        {
+            return false;
+        }
+
+        try
+        {
+            await _session.LoadAsync(_sessionPath, cancellationToken);
+            AnsiConsole.MarkupLine($"[dim grey]Auto-loaded previous session.[/]");
+
+            // Print the last message if there is one
+            if (_session.History.Count > 1)
+            {
+                var lastMessage = _session.History[^1];
+                var preview = lastMessage.Text.Length > 280
+                    ? lastMessage.Text[..277] + "..."
+                    : lastMessage.Text;
+                AnsiConsole.MarkupLine($"[dim grey]  Last message: {Markup.Escape(preview)}[/]");
+            }
+
+            return true;
+        }
+        catch
+        {
+            // Ignore load errors
+            return false;
+        }
+    }
+
+    /// <summary>
+    /// Attempts to save the session to disk. Returns true if successful.
+    /// </summary>
+    public async Task<bool> TrySaveAsync(CancellationToken cancellationToken = default)
+    {
+        try
+        {
+            var directory = Path.GetDirectoryName(_sessionPath);
+            if (!string.IsNullOrEmpty(directory) && !Directory.Exists(directory))
+            {
+                Directory.CreateDirectory(directory);
+            }
+
+            await _session.SaveAsync(_sessionPath, cancellationToken);
+            return true;
+        }
+        catch
+        {
+            // Ignore save errors
+            return false;
+        }
+    }
+
+    /// <summary>
+    /// Saves the session after an LLM turn completes.
+    /// </summary>
+    public async Task SaveAfterTurnAsync(CancellationToken cancellationToken = default)
+    {
+        await TrySaveAsync(cancellationToken);
+    }
+}