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.

tests/ClaudeDo.Worker.Tests/WaitingForChildrenLifecycleTests.cs

@@ -183,6 +183,37 @@ public sealed class WaitingForChildrenLifecycleTests : IDisposable
         }
     }
 
+    [Fact]
+    public async Task Cancelling_WaitingForChildren_parent_cascades_to_nonterminal_children()
+    {
+        using (var ctx = _db.CreateContext())
+        {
+            ctx.Lists.Add(new ListEntity { Id = "l1", Name = "L", CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity { Id = "par", ListId = "l1", Title = "Parent",
+                Status = TaskStatus.WaitingForChildren, CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity { Id = "c_running", ListId = "l1", Title = "Running child",
+                Status = TaskStatus.Running, ParentTaskId = "par", CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity { Id = "c_queued", ListId = "l1", Title = "Queued child",
+                Status = TaskStatus.Queued, ParentTaskId = "par", BlockedByTaskId = "c_running",
+                CreatedAt = DateTime.UtcNow });
+            await ctx.SaveChangesAsync();
+        }
+
+        var result = await _built.State.CancelAsync("par", DateTime.UtcNow, default);
+
+        Assert.True(result.Ok);
+        using var ctx2 = _db.CreateContext();
+        var repo = new TaskRepository(ctx2);
+        var parent = await repo.GetByIdAsync("par");
+        var runningChild = await repo.GetByIdAsync("c_running");
+        var queuedChild = await repo.GetByIdAsync("c_queued");
+
+        Assert.Equal(TaskStatus.Cancelled, parent!.Status);
+        Assert.Equal(TaskStatus.Cancelled, runningChild!.Status);
+        Assert.Equal(TaskStatus.Cancelled, queuedChild!.Status);
+        Assert.Null(queuedChild.BlockedByTaskId);
+    }
+
     // ─── FinalizePlanningAsync ────────────────────────────────────────────
 
     private async Task<string> SeedActivePlanningParentAsync(string id = "par")