docs(worker): clarify SetTaskConfig null-clears-override wording

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Worker/External/ConfigMcpTools.cs

@@ -51,7 +51,7 @@ public sealed class ConfigMcpTools
         await _broadcaster.ListUpdated(listId);
     }
 
-    [McpServerTool, Description("Set per-task config overrides (model/system prompt/agent path). Pass null to clear a field.")]
+    [McpServerTool, Description("Set per-task config overrides (model/system prompt/agent path). Pass null for any field to clear that override.")]
     public async Task SetTaskConfig(
         string taskId, string? model, string? systemPrompt, string? agentPath, CancellationToken cancellationToken)
     {