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/Program.cs

@@ -27,6 +27,7 @@ builder.Services.AddDbContextFactory<ClaudeDoDbContext>(opt =>
 
 builder.Services.AddSingleton(cfg);
 builder.Services.AddHostedService<StaleTaskRecovery>();
+builder.Services.AddHostedService<OrphanRecovery>();
 builder.Services.AddSignalR().AddJsonProtocol(options =>
 {
     options.PayloadSerializerOptions.Converters.Add(new System.Text.Json.Serialization.JsonStringEnumConverter());