fix(tests): stop Ui.Tests order-dependent flakiness on shared Loc.Current

Loc.Current (src/ClaudeDo.Ui/Localization/Loc.cs) is a process-wide mutable
static; which localizer is active depended on xUnit's parallel test
scheduling. Ui.Tests: disable collection parallelization (cheap, pure VM
suite) and install the real localizer in a module initializer so no test
can ever observe the uninitialized key-echo default. Worker.Tests: install
the same module initializer but leave parallelization on — disabling it
measured a 27% wall-clock regression on its real-SQLite/git suite, and the
initializer alone already removes the race. AgentConfigEditorViewModelTests
now installs its own localizer instead of relying on ambient state.

tests/ClaudeDo.Ui.Tests/TestSetup.cs

@@ -1,5 +1,12 @@
 using System.Runtime.CompilerServices;
+using ClaudeDo.Localization;
+using ClaudeDo.Ui.Localization;
 using ClaudeDo.Ui.Services;
+using Xunit;
+
+// Loc.Current (src/ClaudeDo.Ui/Localization/Loc.cs) is a process-wide mutable static that many
+// ViewModel test constructors assign — parallel collections race on which localizer is active.
+[assembly: CollectionBehavior(DisableTestParallelization = true)]
 
 namespace ClaudeDo.Ui.Tests;
 
@@ -17,4 +24,17 @@ internal static class TestSetup
         var path = Path.Combine(Path.GetTempPath(), "claudedo-ui-tests", $"operation-timing-{Environment.ProcessId}.ndjson");
         OperationTiming.Shared = new OperationTiming(path);
     }
+
+    // Installs the real localizer before any test runs, so a class that reads Loc.T/Loc.Current
+    // without setting it up itself (e.g. via a ViewModel error message) doesn't fall back to
+    // Loc's key-echo default depending on which test class happens to run first.
+    [ModuleInitializer]
+    internal static void InstallRealLocalizer()
+    {
+        var dir = AppContext.BaseDirectory;
+        while (dir is not null && !Directory.Exists(Path.Combine(dir, "src", "ClaudeDo.Localization", "locales")))
+            dir = Path.GetDirectoryName(dir);
+        Loc.Current = new Localizer(
+            LocaleStore.Load(Path.Combine(dir!, "src", "ClaudeDo.Localization", "locales")), "en");
+    }
 }

tests/ClaudeDo.Ui.Tests/ViewModels/AgentConfigEditorViewModelTests.cs

@@ -1,4 +1,5 @@
 using ClaudeDo.Data.Models;
+using ClaudeDo.Localization;
 using ClaudeDo.Ui.Localization;
 using ClaudeDo.Ui.Services;
 using ClaudeDo.Ui.ViewModels.Agent;
@@ -9,6 +10,18 @@ namespace ClaudeDo.Ui.Tests.ViewModels;
 
 public class AgentConfigEditorViewModelTests
 {
+    public AgentConfigEditorViewModelTests()
+    {
+        // Loc.T(...) is read below without installing a localizer via a ViewModel constructor
+        // (unlike its sibling test classes) — install one directly so this class doesn't depend
+        // on whichever neighbour test happened to run first and set Loc.Current.
+        var dir = AppContext.BaseDirectory;
+        while (dir is not null && !Directory.Exists(Path.Combine(dir, "src", "ClaudeDo.Localization", "locales")))
+            dir = Path.GetDirectoryName(dir);
+        Loc.Current = new Localizer(
+            LocaleStore.Load(Path.Combine(dir!, "src", "ClaudeDo.Localization", "locales")), "en");
+    }
+
     private sealed class FakeWorker : StubWorkerClient
     {
         public AppSettingsDto? App;