feat(worker): allow cancelling a WaitingForChildren parent

Add WaitingForChildren to the CancelAsync guard so a parent waiting on its
children can be cancelled.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/ClaudeDo.Worker.Tests/WaitingForChildrenLifecycleTests.cs

@@ -162,6 +162,27 @@ public sealed class WaitingForChildrenLifecycleTests : IDisposable
         }
     }
 
+    [Fact]
+    public async Task WaitingForChildren_parent_can_be_cancelled()
+    {
+        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 });
+            await ctx.SaveChangesAsync();
+        }
+
+        var result = await _built.State.CancelAsync("par", DateTime.UtcNow, default);
+
+        Assert.True(result.Ok);
+        using (var ctx = _db.CreateContext())
+        {
+            var par = await new TaskRepository(ctx).GetByIdAsync("par");
+            Assert.Equal(TaskStatus.Cancelled, par!.Status);
+        }
+    }
+
     // ─── FinalizePlanningAsync ────────────────────────────────────────────
 
     private async Task<string> SeedActivePlanningParentAsync(string id = "par")