test(installer): add InstallManifest wrong-shape json test

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

tests/ClaudeDo.Installer.Tests/InstallManifestStoreTests.cs

@@ -63,4 +63,14 @@ public sealed class InstallManifestStoreTests : IDisposable
         var result = InstallManifestStore.TryRead(_tempDir);
         Assert.Null(result);
     }
+
+    [Fact]
+    public void TryRead_ReturnsNull_WhenJsonIsValidButShapeIsWrong()
+    {
+        // Valid JSON, but installedAt has a wrong type — causes JsonException, swallowed silently.
+        File.WriteAllText(Path.Combine(_tempDir, "install.json"),
+            """{"version":"1.0","installDir":"x","workerDir":"y","installedAt":12345}""");
+        var result = InstallManifestStore.TryRead(_tempDir);
+        Assert.Null(result);
+    }
 }