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.Worker.Tests/UiVm/TasksIslandViewModelPlanningTests.cs

@@ -1,5 +1,6 @@
 using ClaudeDo.Data;
 using ClaudeDo.Data.Models;
+using ClaudeDo.Data.Repositories;
 using ClaudeDo.Ui.Services;
 using ClaudeDo.Ui.ViewModels.Islands;
 using CommunityToolkit.Mvvm.Input;
@@ -45,7 +46,11 @@ sealed class FakeWorkerClient : IWorkerClient
     public Task OpenInteractiveTerminalAsync(string taskId, CancellationToken ct = default) => Task.CompletedTask;
     public Task QueuePlanningSubtasksAsync(string parentTaskId, CancellationToken ct = default) => Task.CompletedTask;
     public Task ResumePlanningSessionAsync(string taskId, CancellationToken ct = default) { ResumePlanningCalls++; return Task.CompletedTask; }
-    public Task DiscardPlanningSessionAsync(string taskId, CancellationToken ct = default) { DiscardPlanningCalls++; return Task.CompletedTask; }
+    public Task<DiscardPlanningOutcome> DiscardPlanningSessionAsync(string taskId, bool dequeueQueuedChildren = false, CancellationToken ct = default)
+    {
+        DiscardPlanningCalls++;
+        return Task.FromResult(new DiscardPlanningOutcome(DiscardPlanningResult.Discarded, 0, 0));
+    }
     public Task FinalizePlanningSessionAsync(string taskId, bool queueAgentTasks = true, CancellationToken ct = default) { FinalizePlanningCalls++; return Task.CompletedTask; }
     public Task<int> GetPendingDraftCountAsync(string taskId, CancellationToken ct = default) => Task.FromResult(0);