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/PromptFiles.cs

@@ -168,6 +168,12 @@ public static class PromptFiles
         - sonnet — normal coding work; the sensible default when unsure.
         - opus — only for genuinely complex, cross-cutting, or hard-to-debug work.
         Do not default everything to opus — most subtasks are haiku or sonnet.
+
+        Only for a subtask you can tell will need noticeably more turns than the
+        default budget (a large or sprawling piece of work), also pass CreateChildTask's
+        `maxTurns` argument with a generous turn count, so the run doesn't die mid-work
+        at the turn limit. Leave `maxTurns` null for everything else — it inherits the
+        list/global default.
         """;
 
     private const string PlanningInitialDefault = """