feat(planning): let CreateChildTask set maxTurns on child tasks

Planning sessions could already steer a subtask's model but not its turn
budget, so a visibly large subtask would still die at the global default
turn limit. maxTurns is optional (default null = inherit list/global
default, matching model); 0/negative values are rejected as invalid input,
consistent with the existing model-alias validation.

src/ClaudeDo.Worker/Planning/PlanningMcpService.cs

@@ -41,16 +41,18 @@ public sealed class PlanningMcpService
         "Create a new draft child task under the current planning session's parent task. " +
         "Set model to the cheapest model that can do this subtask well — 'haiku' for trivial/mechanical " +
         "work, 'sonnet' for normal coding (the default), 'opus' only for complex or cross-cutting work. " +
-        "Leave model null to inherit the list/global default.")]
+        "Leave model null to inherit the list/global default. Set maxTurns only when this subtask is " +
+        "visibly larger than the default turn budget; leave it null to inherit the list/global default.")]
     public async Task<CreatedChildDto> CreateChildTask(
         string title,
         string? description,
         string? commitType,
         string? model,
-        CancellationToken cancellationToken)
+        int? maxTurns = null,
+        CancellationToken cancellationToken = default)
     {
         var ctx = _contextAccessor.Current;
-        var child = await _tasks.CreateChildAsync(ctx.ParentTaskId, title, description, commitType, createdBy: null, model: model, ct: cancellationToken);
+        var child = await _tasks.CreateChildAsync(ctx.ParentTaskId, title, description, commitType, createdBy: null, model: model, maxTurns: maxTurns, ct: cancellationToken);
         await BroadcastTaskUpdatedAsync(child.Id, cancellationToken);
         await BroadcastTaskUpdatedAsync(ctx.ParentTaskId, cancellationToken);
         return new CreatedChildDto(child.Id, child.Status.ToString());