feat(installer): add Diagnose section to SettingsWindow

Re-runs the environment checks against the installed configuration
(worker.config.json + detected install dir) without blocking navigation
and without auto-running on window open, only on a "Recheck" click.

Extracted the check-row rendering and check-run logic (busy state,
summary, Recheck command) out of SystemCheckPage into a shared
Checks/CheckListViewModel + Checks/CheckListView, composed by both
SystemCheckPage (wizard) and the new DiagnosePage (settings) instead
of duplicating it.

tests/ClaudeDo.Installer.Tests/PageResolverTests.cs

@@ -1,4 +1,6 @@
+using ClaudeDo.Installer.Checks;
 using ClaudeDo.Installer.Core;
+using ClaudeDo.Installer.Pages.DiagnosePage;
 using ClaudeDo.Installer.Pages.PathsPage;
 using ClaudeDo.Installer.Pages.ServicePage;
 using ClaudeDo.Installer.Pages.SystemCheckPage;
@@ -40,4 +42,33 @@ public class PageResolverTests
 
         Assert.False(page.ShowInSettings);
     }
+
+    [Fact]
+    public void DiagnosePage_is_shown_in_settings_but_not_in_the_wizard()
+    {
+        var context = new InstallContext();
+        var page = new DiagnosePageViewModel(context, () => throw new InvalidOperationException(), () => new InstallerWorkerConfig());
+
+        Assert.True(page.ShowInSettings);
+        Assert.False(page.ShowInWizard);
+    }
+
+    [Fact]
+    public void DiagnosePage_appears_in_the_settings_sidebar_after_the_other_settings_pages()
+    {
+        var context = new InstallContext();
+        var pages = new IInstallerPage[]
+        {
+            new PathsPageViewModel(context),
+            new ServicePageViewModel(context),
+            new UiSettingsPageViewModel(context),
+            new DiagnosePageViewModel(context, () => throw new InvalidOperationException(), () => new InstallerWorkerConfig()),
+        };
+
+        var resolver = new PageResolver(pages);
+        var settingsPages = resolver.SettingsPages.ToList();
+
+        Assert.Equal(4, settingsPages.Count);
+        Assert.IsType<DiagnosePageViewModel>(settingsPages[^1]);
+    }
 }