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

src/ClaudeDo.Installer/Core/InstallManifest.cs

@@ -0,0 +1,48 @@
+using System.IO;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace ClaudeDo.Installer.Core;
+
+public sealed record InstallManifest(
+    string Version,
+    string InstallDir,
+    string WorkerDir,
+    DateTimeOffset InstalledAt);
+
+public static class InstallManifestStore
+{
+    public const string FileName = "install.json";
+
+    private static readonly JsonSerializerOptions JsonOptions = new()
+    {
+        WriteIndented = true,
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+    };
+
+    public static string ManifestPath(string installDir) => Path.Combine(installDir, FileName);
+
+    public static InstallManifest? TryRead(string installDir)
+    {
+        var path = ManifestPath(installDir);
+        if (!File.Exists(path)) return null;
+
+        try
+        {
+            var json = File.ReadAllText(path);
+            return JsonSerializer.Deserialize<InstallManifest>(json, JsonOptions);
+        }
+        catch
+        {
+            return null;
+        }
+    }
+
+    public static void Write(string installDir, InstallManifest manifest)
+    {
+        Directory.CreateDirectory(installDir);
+        var json = JsonSerializer.Serialize(manifest, JsonOptions);
+        File.WriteAllText(ManifestPath(installDir), json);
+    }
+}