fix(worker): advance parent when the last non-terminal child is deleted

DeleteTask never re-evaluated a WaitingForChildren parent, so deleting
the last non-terminal child left it stuck (the BlockedByTaskId SET NULL
FK only repairs the blocked chain, not parent status). Expose
TaskStateService.TryAdvanceParentAsync on the interface and call it
from ExternalMcpService.DeleteTask after a child delete.

tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs

@@ -104,7 +104,8 @@ public sealed class ExternalMcpServiceTests : IDisposable
         return id;
     }
 
-    private async Task<TaskEntity> SeedTaskAsync(string listId, string title = "t", TaskStatus status = TaskStatus.Idle)
+    private async Task<TaskEntity> SeedTaskAsync(
+        string listId, string title = "t", TaskStatus status = TaskStatus.Idle, string? parentId = null)
     {
         var task = new TaskEntity
         {
@@ -112,6 +113,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
             ListId = listId,
             Title = title,
             Status = status,
+            ParentTaskId = parentId,
             CreatedAt = DateTime.UtcNow,
             CommitType = "chore",
         };
@@ -284,6 +286,21 @@ public sealed class ExternalMcpServiceTests : IDisposable
             sut.DeleteTask("does-not-exist", CancellationToken.None));
     }
 
+    [Fact]
+    public async Task DeleteTask_LastNonTerminalChild_AdvancesParentToWaitingForReview()
+    {
+        var listId = await SeedListAsync();
+        var parent = await SeedTaskAsync(listId, status: TaskStatus.WaitingForChildren);
+        await SeedTaskAsync(listId, status: TaskStatus.Done, parentId: parent.Id);
+        var lastChild = await SeedTaskAsync(listId, status: TaskStatus.Idle, parentId: parent.Id);
+        var sut = BuildSut(CreateQueue());
+
+        await sut.DeleteTask(lastChild.Id, CancellationToken.None);
+
+        var reloadedParent = await _tasks.GetByIdAsync(parent.Id);
+        Assert.Equal(TaskStatus.WaitingForReview, reloadedParent!.Status);
+    }
+
     private ExternalMcpService NewService() => BuildSut(CreateQueue());
 
     private async Task<string> SeedIdleTask(string title = "t")