fix(ui): warn and block save when a list's working dir isn't a git repo

Reuses RepoScanner.IsRepo (now public) to validate WorkingDir whenever it
changes in ListSettingsModalViewModel — Browse or typing. A missing folder
or a non-repo folder shows an inline warning and gates Save via the same
CanExecute pattern the load-failure gate already uses; an empty WorkingDir
is untouched. Previously this only surfaced as a Failed task deep in
WorktreeManager on first run.

Also localizes the two hardcoded EN strings in the same file (delete
confirm, planning-sessions block) into vm.listSettings.* keys.

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

@@ -161,7 +161,9 @@ public class ListSettingsModalViewModelTests
         string? reported = null;
         vm.ErrorReported += m => reported = m;
 
-        await vm.LoadAsync("l1", "List One", "/repo", "feat");
+        // Empty working dir: this test is about ticket-project load isolation, not the
+        // working-dir-is-a-repo gate added for the "not a git repo" Save check.
+        await vm.LoadAsync("l1", "List One", "", "feat");
 
         Assert.Equal("make test", vm.VerifyCommand);
         Assert.True(vm.PowerUseAllowed);
@@ -169,4 +171,45 @@ public class ListSettingsModalViewModelTests
         Assert.NotNull(reported);
         Assert.Contains("ticket system down", reported);
     }
+
+    [Fact]
+    public async Task LoadAsync_WorkingDirNotAGitRepo_WarnsAndBlocksSave()
+    {
+        var plainDir = Directory.CreateTempSubdirectory().FullName;
+        var worker = new CountingWorker { Cfg = new ListConfigDto("opus", null, null) };
+        var vm = new ListSettingsModalViewModel(worker, NewFactory());
+
+        await vm.LoadAsync("l1", "List One", plainDir, "feat");
+
+        Assert.NotEmpty(vm.WorkingDirWarning);
+        Assert.False(vm.SaveCommand.CanExecute(null));
+    }
+
+    [Fact]
+    public async Task LoadAsync_WorkingDirIsAGitRepo_NoWarningAndSaveAllowed()
+    {
+        var repoDir = Directory.CreateTempSubdirectory().FullName;
+        Directory.CreateDirectory(Path.Combine(repoDir, ".git"));
+        var worker = new CountingWorker { Cfg = new ListConfigDto("opus", null, null) };
+        var vm = new ListSettingsModalViewModel(worker, NewFactory());
+
+        await vm.LoadAsync("l1", "List One", repoDir, "feat");
+
+        Assert.Empty(vm.WorkingDirWarning);
+        Assert.True(vm.SaveCommand.CanExecute(null));
+    }
+
+    [Fact]
+    public void WorkingDir_ClearedAfterNotARepoWarning_ClearsWarningAndAllowsSave()
+    {
+        var plainDir = Directory.CreateTempSubdirectory().FullName;
+        var worker = new CountingWorker { Cfg = new ListConfigDto("opus", null, null) };
+        var vm = new ListSettingsModalViewModel(worker, NewFactory()) { WorkingDir = plainDir };
+        Assert.NotEmpty(vm.WorkingDirWarning);
+
+        vm.WorkingDir = "";
+
+        Assert.Empty(vm.WorkingDirWarning);
+        Assert.True(vm.SaveCommand.CanExecute(null));
+    }
 }