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/Services/WorkerClient.cs

@@ -433,15 +433,20 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
         }
     }
 
-    public async Task<bool> SetWorktreeStateAsync(string taskId, WorktreeState newState)
+    public async Task<(bool Ok, string? Error)> SetWorktreeStateAsync(string taskId, WorktreeState newState)
     {
         try
         {
-            return await _hub.InvokeAsync<bool>("SetWorktreeState", taskId, newState);
+            var ok = await _hub.InvokeAsync<bool>("SetWorktreeState", taskId, newState);
+            return (ok, null);
         }
-        catch
+        catch (HubException ex)
         {
-            return false;
+            return (false, ex.Message);
+        }
+        catch (Exception)
+        {
+            return (false, "Worker offline.");
         }
     }