ci: gate on the fast tests (about 20s); slow end-to-end tests run in parallel with the image build [CI]

backend/tests/MoShiro.Backend.Tests/OnDemandTestsTests.cs

@@ -67,6 +67,7 @@ public sealed class OnDemandTestsTests
     }
 
     [Fact]
+    [Trait("Category", "Slow")]
     public async Task Hy2Ladder_RunsEndToEnd_AndProducesAResultWithScores()
     {
         await using AgentSimulator agent = await AgentWithProbeAsync();