feat(installer): add WriteInstallManifestStep

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

tests/ClaudeDo.Installer.Tests/WriteInstallManifestStepTests.cs

@@ -0,0 +1,51 @@
+using ClaudeDo.Installer.Core;
+using ClaudeDo.Installer.Steps;
+
+namespace ClaudeDo.Installer.Tests;
+
+public sealed class WriteInstallManifestStepTests : IDisposable
+{
+    private readonly string _installDir;
+
+    public WriteInstallManifestStepTests()
+    {
+        _installDir = Path.Combine(Path.GetTempPath(), "ClaudeDoWriteManifest-" + Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(_installDir);
+    }
+
+    public void Dispose()
+    {
+        try { Directory.Delete(_installDir, recursive: true); } catch { }
+    }
+
+    [Fact]
+    public async Task Writes_Manifest_WithAllFields()
+    {
+        var ctx = new InstallContext
+        {
+            InstallDirectory = _installDir,
+            InstalledVersion = "0.2.0",
+        };
+
+        var step = new WriteInstallManifestStep();
+        var result = await step.ExecuteAsync(ctx, new Progress<string>(_ => { }), CancellationToken.None);
+
+        Assert.True(result.Success);
+        var manifest = InstallManifestStore.TryRead(_installDir);
+        Assert.NotNull(manifest);
+        Assert.Equal("0.2.0", manifest!.Version);
+        Assert.Equal(_installDir, manifest.InstallDir);
+        Assert.Equal(Path.Combine(_installDir, "worker"), manifest.WorkerDir);
+    }
+
+    [Fact]
+    public async Task Fails_When_InstalledVersion_Missing()
+    {
+        var ctx = new InstallContext { InstallDirectory = _installDir };  // no version set
+
+        var step = new WriteInstallManifestStep();
+        var result = await step.ExecuteAsync(ctx, new Progress<string>(_ => { }), CancellationToken.None);
+
+        Assert.False(result.Success);
+    }
+}