chore(claude-do): merge [C4] Rebase-after-Merge sichtbar machen + WorktreeMaintenanc

ClaudeDo-Task: 0a64e32d-6308-4f2d-a69e-fb8b3cabd01a

src/ClaudeDo.Worker/Lifecycle/TaskMergeService.cs

@@ -64,6 +64,7 @@ public sealed class TaskMergeService
     // Phase tokens for the OperationProgress broadcast — stable identifiers, localized by the UI.
     public const string PhaseMerging   = "merging";
     public const string PhaseVerifying = "verifying";
+    public const string PhaseRebasing  = "rebasing";
 
     public const string PreviewClean       = "clean";
     public const string PreviewConflict    = "conflict";
@@ -296,9 +297,15 @@ public sealed class TaskMergeService
             var tasks = await new TaskRepository(ctx).GetByListIdAsync(list.Id, ct);
             candidates = tasks.Where(t => t.Id != mergedTask.Id && t.Status == TaskStatus.WaitingForReview).ToList();
         }
+        if (candidates.Count == 0) return;
 
-        foreach (var candidate in candidates)
-            await RebaseOneIfOverlappingAsync(candidate, targetBranch, newTargetTip, landedFiles, ct);
+        // Runs where the stalled "Merging…" phase would otherwise sit unchanged in the UI while
+        // this loop does real (if best-effort) work — see the correction note in the task spec.
+        for (var i = 0; i < candidates.Count; i++)
+        {
+            await _broadcaster.OperationProgress(mergedTask.Id, PhaseRebasing, i + 1, candidates.Count);
+            await RebaseOneIfOverlappingAsync(candidates[i], targetBranch, newTargetTip, landedFiles, ct);
+        }
     }
 
     private async Task RebaseOneIfOverlappingAsync(