fix(ui): failureReason "usage_limit" nachziehen + Worktree-Gate prüft die Platte

Zwei liegengebliebene Consumer aus den letzten beiden Commits:

- TaskRunner klassifiziert seit 07dd7570 "usage_limit", aber weder
  TaskRowViewModel.FailureReasonLabel noch vm.failureReason (de/en) noch
  die get_task-Tool-Beschreibung kannten den Wert — die UI zeigte
  "Grund unbekannt", das MCP-Doc listete weiterhin max_turns|timeout|error.
- TaskRowViewModel.CanOpenWorktree prüfte nur auf einen nicht-leeren
  String. Die Zeile behält den Path eines gemergten/verworfenen Worktrees,
  also war der Menüpunkt aktiv und Process.Start warf in den Footer.
  Jetzt zusätzlich Directory.Exists — dieselbe Prüfung, die
  WorktreesOverviewModalViewModel und MergeSectionViewModel schon machen.

tests/ClaudeDo.Worker.Tests/UiVm/TaskRowViewModelTests.cs

@@ -65,13 +65,19 @@ public class TaskRowViewModelTests
     }
 
     [Fact]
-    public void CanOpenWorktree_True_And_ReasonNull_When_Path_Present()
+    public void CanOpenWorktree_True_And_ReasonNull_When_Folder_Exists()
     {
-        var vm = new TaskRowViewModel { Id = "t" };
-        vm.WorktreePath = @"C:\repo\worktrees\t";
+        var dir = Path.Combine(Path.GetTempPath(), $"cd_wt_{Guid.NewGuid():N}");
+        Directory.CreateDirectory(dir);
+        try
+        {
+            var vm = new TaskRowViewModel { Id = "t" };
+            vm.WorktreePath = dir;
 
-        Assert.True(vm.CanOpenWorktree);
-        Assert.Null(vm.OpenWorktreeDisabledReason);
+            Assert.True(vm.CanOpenWorktree);
+            Assert.Null(vm.OpenWorktreeDisabledReason);
+        }
+        finally { Directory.Delete(dir, true); }
     }
 
     [Theory]
@@ -87,6 +93,18 @@ public class TaskRowViewModelTests
         Assert.NotNull(vm.OpenWorktreeDisabledReason);
     }
 
+    // A merged/discarded worktree leaves its recorded Path on the row long after the folder is
+    // gone — the menu entry has to gray out, not throw inside Process.Start.
+    [Fact]
+    public void CanOpenWorktree_False_When_Recorded_Folder_Is_Gone()
+    {
+        var vm = new TaskRowViewModel { Id = "t" };
+        vm.WorktreePath = Path.Combine(Path.GetTempPath(), $"cd_wt_gone_{Guid.NewGuid():N}");
+
+        Assert.False(vm.CanOpenWorktree);
+        Assert.NotNull(vm.OpenWorktreeDisabledReason);
+    }
+
     [Fact]
     public void OpenWorktreeGate_Raises_PropertyChanged_On_WorktreePath_Change()
     {
@@ -115,7 +133,6 @@ public class TaskRowViewModelTests
         var vm = TaskRowViewModel.FromEntity(entity);
 
         Assert.Equal(@"C:\repo\worktrees\t", vm.WorktreePath);
-        Assert.True(vm.CanOpenWorktree);
     }
 
     [Fact]