feat(ui): merge action and robust jump-to-task in worktrees overview

Add Merge entry to the worktrees overview context menu wiring the existing
MergeModalViewModel, replace fire-and-forget list selection with a
collection-change-aware JumpToTaskHelper, and propagate list renames to
visible task rows via a new ListUpdated event.

Harden worktree state changes: WorkerHub.SetWorktreeState now rejects
invalid transitions, WorktreeMaintenanceService only drops the DB row when
the on-disk worktree was actually removed, and Cleanup/Reset broadcast
WorktreeUpdated for affected tasks. SetWorktreeStateAsync returns the hub
error message so the modal can surface it.

Also: de-duplicate the worktrees overview modal opener, hook
OnParentTaskIdChanged to refresh IsDraft, fix MergeModal CanExecute
notifications, and add WorktreeStateHubTests for the transition rules.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/JumpToTaskHelper.cs

@@ -0,0 +1,42 @@
+using System.Collections.Specialized;
+using System.Linq;
+using System.Threading.Tasks;
+using ClaudeDo.Ui.ViewModels;
+
+namespace ClaudeDo.Ui.Views;
+
+internal static class JumpToTaskHelper
+{
+    // Selects the list, then waits for the matching task row to appear in Tasks.Items.
+    // Listens to CollectionChanged so it works regardless of how long LoadForListAsync takes;
+    // bounded by a hard timeout so we never hang if the task is filtered out.
+    public static async Task SelectAsync(IslandsShellViewModel s, string listId, string taskId)
+    {
+        if (s.Lists is null || s.Tasks is null) return;
+        var item = s.Lists.UserLists.FirstOrDefault(l => l.Id == $"user:{listId}");
+        if (item is null) return;
+        s.Lists.SelectedList = item;
+
+        var existing = s.Tasks.Items.FirstOrDefault(r => r.Id == taskId);
+        if (existing is not null) { s.Tasks.SelectedTask = existing; return; }
+
+        var tcs = new TaskCompletionSource<bool>();
+        void OnChanged(object? _, NotifyCollectionChangedEventArgs __)
+        {
+            var row = s.Tasks!.Items.FirstOrDefault(r => r.Id == taskId);
+            if (row is not null) tcs.TrySetResult(true);
+        }
+
+        s.Tasks.Items.CollectionChanged += OnChanged;
+        try
+        {
+            await Task.WhenAny(tcs.Task, Task.Delay(5000));
+            var row = s.Tasks.Items.FirstOrDefault(r => r.Id == taskId);
+            if (row is not null) s.Tasks.SelectedTask = row;
+        }
+        finally
+        {
+            s.Tasks.Items.CollectionChanged -= OnChanged;
+        }
+    }
+}