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>

src/ClaudeDo.Worker/Planning/PlanningSessionManager.cs

@@ -236,12 +236,17 @@ public sealed class PlanningSessionManager
         return children.Count(c => c.Status == TaskStatus.Idle);
     }
 
-    public async Task DiscardAsync(string taskId, CancellationToken ct)
+    public async Task<DiscardPlanningOutcome> DiscardAsync(
+        string taskId,
+        bool dequeueQueuedChildren,
+        CancellationToken ct)
     {
         var (tasks, lists, settings, ctx) = CreateRepos();
         await using var __ = ctx;
 
-        var ok = await tasks.DiscardPlanningAsync(taskId, ct);
+        var outcome = await tasks.DiscardPlanningAsync(taskId, dequeueQueuedChildren, ct);
+        if (outcome.Result != DiscardPlanningResult.Discarded)
+            return outcome;
 
         await TryCleanupWorktreeAsync(taskId, lists, settings, ct);
 
@@ -251,8 +256,7 @@ public sealed class PlanningSessionManager
             try { Directory.Delete(sessionDir, recursive: true); } catch { }
         }
 
-        if (!ok)
-            throw new InvalidOperationException($"Task {taskId} was not in Planning state; nothing to discard.");
+        return outcome;
     }
 
     public async Task<PlanningSessionResumeContext> ResumeAsync(string taskId, CancellationToken ct)