feat(installer): add AutostartShortcut helper for Startup-folder lnk

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/ClaudeDo.Installer.Tests/AutostartShortcutTests.cs

@@ -0,0 +1,57 @@
+using System.IO;
+using ClaudeDo.Installer.Core;
+
+namespace ClaudeDo.Installer.Tests;
+
+public class AutostartShortcutTests
+{
+    private static string TempDir()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), "cdautostart-" + Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(dir);
+        return dir;
+    }
+
+    [Fact]
+    public void Install_creates_lnk_with_expected_name()
+    {
+        var startup = TempDir();
+        var workerDir = TempDir();
+        try
+        {
+            var workerExe = Path.Combine(workerDir, "ClaudeDo.Worker.exe");
+            File.WriteAllText(workerExe, "");
+
+            AutostartShortcut.Install(startup, workerExe);
+
+            Assert.True(File.Exists(Path.Combine(startup, AutostartShortcut.FileName)));
+        }
+        finally { Directory.Delete(startup, true); Directory.Delete(workerDir, true); }
+    }
+
+    [Fact]
+    public void Remove_deletes_existing_lnk()
+    {
+        var startup = TempDir();
+        var workerDir = TempDir();
+        try
+        {
+            var workerExe = Path.Combine(workerDir, "ClaudeDo.Worker.exe");
+            File.WriteAllText(workerExe, "");
+            AutostartShortcut.Install(startup, workerExe);
+
+            AutostartShortcut.Remove(startup);
+
+            Assert.False(File.Exists(Path.Combine(startup, AutostartShortcut.FileName)));
+        }
+        finally { Directory.Delete(startup, true); Directory.Delete(workerDir, true); }
+    }
+
+    [Fact]
+    public void Remove_is_noop_when_missing()
+    {
+        var startup = TempDir();
+        try { AutostartShortcut.Remove(startup); } // must not throw
+        finally { Directory.Delete(startup, true); }
+    }
+}