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/Repositories/TaskRepositoryPlanningTests.cs

@@ -205,9 +205,9 @@ public sealed class TaskRepositoryPlanningTests : IDisposable
         var c1 = await _tasks.CreateChildAsync(parent.Id, "c1", null, null, null);
         var c2 = await _tasks.CreateChildAsync(parent.Id, "c2", null, null, null);
 
-        var ok = await _tasks.DiscardPlanningAsync(parent.Id);
+        var outcome = await _tasks.DiscardPlanningAsync(parent.Id, dequeueQueuedChildren: false);
 
-        Assert.True(ok);
+        Assert.Equal(DiscardPlanningResult.Discarded, outcome.Result);
         Assert.Null(await _tasks.GetByIdAsync(c1.Id));
         Assert.Null(await _tasks.GetByIdAsync(c2.Id));
 
@@ -226,9 +226,9 @@ public sealed class TaskRepositoryPlanningTests : IDisposable
         var task = MakeTask(listId);
         await _tasks.AddAsync(task);
 
-        var ok = await _tasks.DiscardPlanningAsync(task.Id);
+        var outcome = await _tasks.DiscardPlanningAsync(task.Id, dequeueQueuedChildren: false);
 
-        Assert.False(ok);
+        Assert.Equal(DiscardPlanningResult.NotInPlanning, outcome.Result);
     }
 
     [Fact]