feat(worker): let Claude set the cheapest model per generated task via MCP

AddTask, planning CreateChildTask, and SuggestImprovement now accept an
optional alias-validated model (haiku/sonnet/opus; blank = inherit) so the
model is chosen at creation time instead of a follow-up set_task_config call.
The planning, system, and improvement prompts instruct Claude to pick the
cheapest capable model (haiku < sonnet < opus).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Worker/Planning/PlanningMcpService.cs

@@ -37,15 +37,20 @@ public sealed class PlanningMcpService
     private Task BroadcastTaskUpdatedAsync(string taskId, CancellationToken ct)
         => _broadcaster.TaskUpdated(taskId);
 
-    [McpServerTool, Description("Create a new draft child task under the current planning session's parent task.")]
+    [McpServerTool, Description(
+        "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.")]
     public async Task<CreatedChildDto> CreateChildTask(
         string title,
         string? description,
         string? commitType,
+        string? model,
         CancellationToken cancellationToken)
     {
         var ctx = _contextAccessor.Current;
-        var child = await _tasks.CreateChildAsync(ctx.ParentTaskId, title, description, commitType, createdBy: null, cancellationToken);
+        var child = await _tasks.CreateChildAsync(ctx.ParentTaskId, title, description, commitType, createdBy: null, model: model, ct: cancellationToken);
         await BroadcastTaskUpdatedAsync(child.Id, cancellationToken);
         await BroadcastTaskUpdatedAsync(ctx.ParentTaskId, cancellationToken);
         return new CreatedChildDto(child.Id, child.Status.ToString());