fix(worker): cascade cancel of a WaitingForChildren parent to its non-terminal children

Confirmed via docs/open.md (Korrektheits-Review 2026-06-09): TaskStateService.CancelAsync
only flipped the target row to Cancelled - a Queued/Running child kept going and could
still commit into its worktree after the parent was gone.

CancelAsync now also cancels the task's own non-terminal children (Idle/Queued/Running/
WaitingForReview/WaitingForChildren) in the same pass, clearing BlockedByTaskId so no
successor is left wedged, and broadcasts TaskUpdated for each.

src/ClaudeDo.Worker/State/TaskStateService.cs

@@ -220,6 +220,7 @@ public sealed class TaskStateService : ITaskStateService
 
     public async Task<TransitionResult> CancelAsync(string taskId, DateTime finishedAt, CancellationToken ct)
     {
+        List<string> cancelledChildIds;
         await using (var ctx = await _dbFactory.CreateDbContextAsync(ct))
         {
             var affected = await ctx.Tasks
@@ -233,10 +234,31 @@ public sealed class TaskStateService : ITaskStateService
 
             if (affected == 0)
                 return new TransitionResult(false, "Task not in cancellable state.");
+
+            // Cascade to this task's own non-terminal children (one level — children are
+            // leaves in the current model) so a Queued/Running child doesn't keep going
+            // against a parent that's already gone, orphaning its worktree commit.
+            cancelledChildIds = await ctx.Tasks
+                .Where(t => t.ParentTaskId == taskId
+                    && t.Status != TaskStatus.Done
+                    && t.Status != TaskStatus.Failed
+                    && t.Status != TaskStatus.Cancelled)
+                .Select(t => t.Id)
+                .ToListAsync(ct);
+
+            if (cancelledChildIds.Count > 0)
+                await ctx.Tasks
+                    .Where(t => cancelledChildIds.Contains(t.Id))
+                    .ExecuteUpdateAsync(s => s
+                        .SetProperty(t => t.Status, TaskStatus.Cancelled)
+                        .SetProperty(t => t.FinishedAt, finishedAt)
+                        .SetProperty(t => t.BlockedByTaskId, (string?)null), ct);
         }
 
         await OnChildTerminalAsync(taskId, TaskStatus.Cancelled);
         await _broadcaster.TaskUpdated(taskId);
+        foreach (var childId in cancelledChildIds)
+            await _broadcaster.TaskUpdated(childId);
         return new TransitionResult(true, null);
     }