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.Data/Repositories/TaskRepository.cs

@@ -276,8 +276,12 @@ public sealed class TaskRepository
         string? commitType,
         string? createdBy = null,
         string? model = null,
+        int? maxTurns = null,
         CancellationToken ct = default)
     {
+        if (maxTurns.HasValue && maxTurns.Value <= 0)
+            throw new ArgumentException($"maxTurns must be positive, got {maxTurns.Value}.");
+
         // AsNoTracking: SetPlanningStartedAsync mutates via ExecuteUpdate which
         // bypasses the change tracker; a tracked Find would return stale data.
         var parent = await _context.Tasks.AsNoTracking()
@@ -303,6 +307,7 @@ public sealed class TaskRepository
             SortOrder = (maxSort ?? -1) + 1,
             CreatedBy = createdBy,
             Model = ModelRegistry.NormalizeAlias(model),
+            MaxTurns = maxTurns,
         };
         _context.Tasks.Add(child);
         await _context.SaveChangesAsync(ct);