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/Runner/WorktreeManager.cs

@@ -256,23 +256,19 @@ public sealed class WorktreeManager
         return TicketStatusMap.ParseBandelTicketId(parentRef);
     }
 
-    // Improvement children (parent is a non-planning task with its own worktree) branch
-    // from the parent's recorded HEAD so they build on the parent's not-yet-merged work.
-    // Planning children and standalone tasks base off list_config.base_branch, else the current HEAD.
+    /// <summary>
+    /// A child of a parent with an Active worktree starts from the unit branch's current head, so it
+    /// sees every sibling absorbed before it. Unit roots (and legacy children) base off
+    /// <c>list_config.base_branch</c>, else the current HEAD.
+    /// </summary>
     private async Task<string> ResolveBaseCommitAsync(TaskEntity task, string workingDir, CancellationToken ct)
     {
         if (task.ParentTaskId is not null)
         {
             using var ctx = _dbFactory.CreateDbContext();
-            var parent = await ctx.Tasks.AsNoTracking()
-                .FirstOrDefaultAsync(t => t.Id == task.ParentTaskId, ct);
-            if (parent is not null && parent.PlanningPhase == PlanningPhase.None)
-            {
-                var parentWt = await new WorktreeRepository(ctx).GetByTaskIdAsync(task.ParentTaskId, ct);
-                var parentHead = parentWt?.HeadCommit ?? parentWt?.BaseCommit;
-                if (parentHead is not null)
-                    return parentHead;
-            }
+            var parentWt = await new WorktreeRepository(ctx).GetByTaskIdAsync(task.ParentTaskId, ct);
+            if (parentWt is { State: WorktreeState.Active })
+                return await _git.RevParseAsync(workingDir, parentWt.BranchName, ct);
         }
         using var cfgCtx = _dbFactory.CreateDbContext();
         var baseBranch = await cfgCtx.ListConfigs.AsNoTracking()