feat(worker): fast-forward finished children into the parent's unit branch

Planning parents get their own worktree on Queue plan, children fork from the
unit head and are absorbed via merge --ff-only before Done (ff failure -> Failed,
interactive child approve -> absorb instead of merge to the target).

src/ClaudeDo.Worker/Lifecycle/TaskMergeService.cs

@@ -117,6 +117,7 @@ public sealed class TaskMergeService
     private readonly ITaskStateService _state;
     private readonly IVerifyCommandRunner _verify;
     private readonly ILogger<TaskMergeService> _logger;
+    private readonly UnitBranchService? _unitBranch;
 
     public TaskMergeService(
         IDbContextFactory<ClaudeDoDbContext> dbFactory,
@@ -124,7 +125,8 @@ public sealed class TaskMergeService
         HubBroadcaster broadcaster,
         ITaskStateService state,
         IVerifyCommandRunner verify,
-        ILogger<TaskMergeService> logger)
+        ILogger<TaskMergeService> logger,
+        UnitBranchService? unitBranch = null)
     {
         _dbFactory = dbFactory;
         _git = git;
@@ -132,6 +134,7 @@ public sealed class TaskMergeService
         _state = state;
         _verify = verify;
         _logger = logger;
+        _unitBranch = unitBranch;
     }
 
     private async Task<(TaskEntity Task, ListEntity List, WorktreeEntity? Worktree, string? VerifyCommand)> LoadMergeContextAsync(
@@ -1010,6 +1013,21 @@ public sealed class TaskMergeService
         if (task.Status != TaskStatus.WaitingForReview)
             return Blocked("task is not waiting for review");
 
+        // An interactive child of a unit lands in the unit branch, not on the target; the parent's
+        // own approve merges the unit. A failed fast-forward keeps the child in review.
+        if (task.ParentTaskId is not null && _unitBranch is not null)
+        {
+            var absorb = await _unitBranch.AbsorbChildAsync(taskId, ct);
+            if (absorb.Failed) return Blocked(absorb.Error!);
+            if (absorb.Applicable)
+            {
+                var approved = await _state.ApproveReviewAsync(taskId, ct);
+                return approved.Ok
+                    ? new MergeResult(StatusMerged, Array.Empty<string>(), null)
+                    : Blocked(approved.Reason ?? "approve failed");
+            }
+        }
+
         if (wt is not null && wt.State == WorktreeState.Kept)
         {
             // Kept means either RevertMergeAsync undid a previous merge, or the user manually