test(installer): cover CanGoNext end-to-end through WizardViewModel navigation

Navigates Welcome -> SystemCheck and asserts the Next button's CanGoNext
flips false on a blocking Error+Failed check and back to true once a
recheck comes back clean, exercising the PropertyChanged wiring between
the page and WizardViewModel rather than just the page's own BlocksNavigation.

tests/ClaudeDo.Installer.Tests/WizardViewModelTests.cs

@@ -1,3 +1,4 @@
+using ClaudeDo.Installer.Checks;
 using ClaudeDo.Installer.Core;
 using ClaudeDo.Installer.Pages.InstallPage;
 using ClaudeDo.Installer.Pages.PathsPage;
@@ -5,6 +6,7 @@ using ClaudeDo.Installer.Pages.ServicePage;
 using ClaudeDo.Installer.Pages.SystemCheckPage;
 using ClaudeDo.Installer.Pages.UiSettingsPage;
 using ClaudeDo.Installer.Pages.WelcomePage;
+using ClaudeDo.Installer.Tests.Pages.SystemCheckPage;
 using ClaudeDo.Installer.Views;
 using Microsoft.Extensions.DependencyInjection;
 
@@ -50,4 +52,35 @@ public class WizardViewModelTests
 
         Assert.DoesNotContain(vm.Pages, p => p is SystemCheckPageViewModel);
     }
+
+    [Fact]
+    public async Task CanGoNext_is_false_while_the_system_check_page_has_a_blocking_error_and_true_after_a_clean_recheck()
+    {
+        var context = new InstallContext { Mode = InstallerMode.FreshInstall };
+        var callNumber = 0;
+        var systemCheckPage = new SystemCheckPageViewModel(context, () =>
+        {
+            callNumber++;
+            IEnvironmentCheck check = callNumber == 1
+                ? FakeEnvironmentCheck.Fail("git", CheckSeverity.Error)
+                : FakeEnvironmentCheck.Ok("git");
+            return new EnvironmentCheckService(new[] { check });
+        });
+
+        var resolver = new PageResolver(new IInstallerPage[]
+        {
+            new WelcomePageViewModel(context),
+            systemCheckPage,
+        });
+        var vm = new WizardViewModel(resolver, context, new FakeLocalizer());
+
+        await vm.GoNextCommand.ExecuteAsync(null); // Welcome -> SystemCheck; auto-run fails
+
+        Assert.Same(systemCheckPage, vm.CurrentPage);
+        Assert.False(vm.CanGoNext);
+
+        await systemCheckPage.RunChecksCommand.ExecuteAsync(null); // recheck comes back clean
+
+        Assert.True(vm.CanGoNext);
+    }
 }