test(installer): pin 'unparseable version = Config' behavior + document IsNewer limits

tests/ClaudeDo.Installer.Tests/InstallModeDetectorTests.cs

@@ -102,4 +102,23 @@ public sealed class InstallModeDetectorTests : IDisposable
 
         Assert.Equal(InstallerMode.Config, state.Mode);
     }
+
+    [Fact]
+    public async Task Detect_Config_WhenInstalledVersion_IsUnparseable()
+    {
+        // install.json has been tampered with or written by an older installer with a
+        // version string we can't compare. Must not crash; must land on Config (no update).
+        InstallManifestStore.Write(_tempDir,
+            new InstallManifest("garbage", _tempDir, _tempDir, DateTimeOffset.UtcNow));
+
+        var fake = new FakeReleaseClient
+        {
+            Release = new GiteaRelease("v0.2.0", "v0.2.0", Array.Empty<ReleaseAsset>())
+        };
+        var detector = new InstallModeDetector(fake);
+
+        var state = await detector.DetectAsync(_tempDir, CancellationToken.None);
+
+        Assert.Equal(InstallerMode.Config, state.Mode);
+    }
 }