fix(installer): harden DownloadAndExtractStep per review

src/ClaudeDo.Installer/Steps/DownloadAndExtractStep.cs

@@ -17,6 +17,9 @@ public sealed class DownloadAndExtractStep : IInstallStep
 
     public async Task<StepResult> ExecuteAsync(InstallContext ctx, IProgress<string> progress, CancellationToken ct)
     {
+        if (string.IsNullOrWhiteSpace(ctx.InstallDirectory))
+            return StepResult.Fail("Install directory is not set.");
+
         progress.Report("Fetching latest release metadata...");
         var release = await _releases.GetLatestReleaseAsync(ct);
         if (release is null)
@@ -24,7 +27,7 @@ public sealed class DownloadAndExtractStep : IInstallStep
 
         var zipAsset = release.Assets.FirstOrDefault(a =>
             a.Name.StartsWith("ClaudeDo-", StringComparison.OrdinalIgnoreCase) &&
-            a.Name.EndsWith(".zip", StringComparison.OrdinalIgnoreCase));
+            a.Name.EndsWith("-win-x64.zip", StringComparison.OrdinalIgnoreCase));
         var checksumAsset = release.Assets.FirstOrDefault(a =>
             a.Name.Equals("checksums.txt", StringComparison.OrdinalIgnoreCase));
 
@@ -51,7 +54,7 @@ public sealed class DownloadAndExtractStep : IInstallStep
                 new Progress<long>(_ => { }), ct);
 
             progress.Report("Verifying checksum...");
-            var map = ChecksumVerifier.ParseChecksumsFile(File.ReadAllText(checksumPath));
+            var map = ChecksumVerifier.ParseChecksumsFile(await File.ReadAllTextAsync(checksumPath, ct));
             if (!map.TryGetValue(zipAsset.Name, out var expectedHash))
                 return StepResult.Fail($"No checksum entry for {zipAsset.Name} in checksums.txt.");
             if (!ChecksumVerifier.Verify(zipPath, expectedHash))
@@ -66,7 +69,16 @@ public sealed class DownloadAndExtractStep : IInstallStep
 
             progress.Report("Extracting...");
             Directory.CreateDirectory(ctx.InstallDirectory);
+            try
+            {
                 ZipFile.ExtractToDirectory(zipPath, ctx.InstallDirectory, overwriteFiles: true);
+            }
+            catch (Exception ex)
+            {
+                return StepResult.Fail(
+                    $"Extraction failed after old binaries were removed: {ex.Message}. " +
+                    "Your install directory may be incomplete. Re-run the installer to retry.");
+            }
 
             ctx.InstalledVersion = release.TagName.TrimStart('v', 'V');
             return StepResult.Ok();

tests/ClaudeDo.Installer.Tests/DownloadAndExtractStepTests.cs

@@ -78,6 +78,7 @@ public sealed class DownloadAndExtractStepTests : IDisposable
         Assert.True(result.Success, result.ErrorMessage);
         Assert.Equal("hello-app", File.ReadAllText(Path.Combine(_installDir, "app", "a.txt")));
         Assert.Equal("hello-worker", File.ReadAllText(Path.Combine(_installDir, "worker", "b.txt")));
+        Assert.Equal("0.1.0", ctx.InstalledVersion);
     }
 
     [Fact]