feat(installer): add IReleaseClient + Gitea ReleaseClient

src/ClaudeDo.Installer/Core/IReleaseClient.cs

@@ -0,0 +1,15 @@
+namespace ClaudeDo.Installer.Core;
+
+public sealed record ReleaseAsset(string Name, string BrowserDownloadUrl, long Size);
+
+public sealed record GiteaRelease(
+    string TagName,
+    string Name,
+    IReadOnlyList<ReleaseAsset> Assets);
+
+public interface IReleaseClient
+{
+    Task<GiteaRelease?> GetLatestReleaseAsync(CancellationToken ct);
+
+    Task DownloadAsync(string url, string destPath, IProgress<long> progress, CancellationToken ct);
+}