feat(planning): prevent orphaned subtasks via guards + startup repair

Three coordinated guards close the orphan-creation paths:

- CreateChildAsync refuses when the parent is not in a planning phase.
- DiscardPlanningAsync now returns a structured DiscardPlanningOutcome
  and refuses when children are queued or running; callers can opt into
  auto-dequeuing queued kids via dequeueQueuedChildren=true. Terminal
  children (Done/Failed/Cancelled) are promoted to top-level instead of
  becoming orphans when the parent's PlanningPhase is reset.
- OrphanRecovery hosted service clears ParentTaskId on any rows whose
  parent is missing or no longer in a planning phase on worker startup,
  mirroring the StaleTaskRecovery pattern.

UI surfaces the block reason: a confirm dialog offers to dequeue queued
children and retry; a running-children block is shown as a hard error
asking the user to cancel first.

WorkerClient now negotiates the JsonStringEnumConverter so the
DiscardPlanningResult enum round-trips correctly over SignalR.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/ClaudeDo.Ui.Tests/ViewModels/PlanningDiffViewModelTests.cs

@@ -39,7 +39,8 @@ public class PlanningDiffViewModelTests
         public Task<List<string>> GetAllTagsAsync() => Task.FromResult(new List<string>());
         public Task StartPlanningSessionAsync(string taskId, CancellationToken ct = default) => Task.CompletedTask;
         public Task ResumePlanningSessionAsync(string taskId, CancellationToken ct = default) => Task.CompletedTask;
-        public Task DiscardPlanningSessionAsync(string taskId, CancellationToken ct = default) => Task.CompletedTask;
+        public Task<ClaudeDo.Data.Repositories.DiscardPlanningOutcome> DiscardPlanningSessionAsync(string taskId, bool dequeueQueuedChildren = false, CancellationToken ct = default)
+            => Task.FromResult(new ClaudeDo.Data.Repositories.DiscardPlanningOutcome(ClaudeDo.Data.Repositories.DiscardPlanningResult.Discarded, 0, 0));
         public Task FinalizePlanningSessionAsync(string taskId, bool queueAgentTasks = true, CancellationToken ct = default) => Task.CompletedTask;
         public Task<int> GetPendingDraftCountAsync(string taskId, CancellationToken ct = default) => Task.FromResult(0);
         public Task<MergeTargetsDto?> GetMergeTargetsAsync(string taskId) => Task.FromResult<MergeTargetsDto?>(null);