Function plan_workflow 

pub async fn plan_workflow(
    task: &str,
    backend: Arc<dyn AgentBackend>,
    cfg: &PlannerConfig,
) -> Result<PlannedWorkflow, PlannerError>

Generate a Lua orchestration script for task by asking the backend agent.

Retries up to cfg.max_retries times, feeding the validation error back to the agent so it can self-correct.