feat(installer): add InstallManifest + json-backed store

tests/ClaudeDo.Installer.Tests/InstallManifestStoreTests.cs

@@ -0,0 +1,66 @@
+using System.IO;
+using ClaudeDo.Installer.Core;
+
+namespace ClaudeDo.Installer.Tests;
+
+public sealed class InstallManifestStoreTests : IDisposable
+{
+    private readonly string _tempDir;
+
+    public InstallManifestStoreTests()
+    {
+        _tempDir = Path.Combine(Path.GetTempPath(), "ClaudeDoInstallerTests-" + Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(_tempDir);
+    }
+
+    public void Dispose()
+    {
+        try { Directory.Delete(_tempDir, recursive: true); } catch { /* best effort */ }
+    }
+
+    [Fact]
+    public void TryRead_ReturnsNull_WhenFileMissing()
+    {
+        var result = InstallManifestStore.TryRead(_tempDir);
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void Write_Then_Read_RoundTripsAllFields()
+    {
+        var manifest = new InstallManifest(
+            Version: "0.2.0",
+            InstallDir: _tempDir,
+            WorkerDir: Path.Combine(_tempDir, "worker"),
+            InstalledAt: new DateTimeOffset(2026, 4, 15, 12, 34, 56, TimeSpan.Zero));
+
+        InstallManifestStore.Write(_tempDir, manifest);
+
+        var round = InstallManifestStore.TryRead(_tempDir);
+        Assert.NotNull(round);
+        Assert.Equal("0.2.0", round!.Version);
+        Assert.Equal(manifest.InstallDir, round.InstallDir);
+        Assert.Equal(manifest.WorkerDir, round.WorkerDir);
+        Assert.Equal(manifest.InstalledAt, round.InstalledAt);
+    }
+
+    [Fact]
+    public void Write_CreatesInstallDir_IfMissing()
+    {
+        var nested = Path.Combine(_tempDir, "nested");
+        Assert.False(Directory.Exists(nested));
+
+        InstallManifestStore.Write(nested, new InstallManifest(
+            "0.0.1", nested, Path.Combine(nested, "worker"), DateTimeOffset.UtcNow));
+
+        Assert.True(File.Exists(Path.Combine(nested, "install.json")));
+    }
+
+    [Fact]
+    public void TryRead_ReturnsNull_WhenJsonMalformed()
+    {
+        File.WriteAllText(Path.Combine(_tempDir, "install.json"), "{ not json");
+        var result = InstallManifestStore.TryRead(_tempDir);
+        Assert.Null(result);
+    }
+}