refactor(installer): extract ShortcutFactory COM helper

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

tests/ClaudeDo.Installer.Tests/ShortcutFactoryTests.cs

@@ -0,0 +1,25 @@
+using System.IO;
+using ClaudeDo.Installer.Core;
+
+namespace ClaudeDo.Installer.Tests;
+
+public class ShortcutFactoryTests
+{
+    [Fact]
+    public void CreateShortcut_writes_lnk_file()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), "cdshortcut-" + Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(dir);
+        try
+        {
+            var target = Path.Combine(dir, "fake.exe");
+            File.WriteAllText(target, "");
+            var lnk = Path.Combine(dir, "x.lnk");
+
+            ShortcutFactory.CreateShortcut(lnk, target, dir, "desc");
+
+            Assert.True(File.Exists(lnk));
+        }
+        finally { Directory.Delete(dir, recursive: true); }
+    }
+}