Function execute 

pub async fn execute(
    db: &DbPool,
    x_client: Option<&dyn XApiClient>,
    llm: Option<&Arc<dyn LlmProvider>>,
    config: &Config,
    input: QueueInput,
) -> Result<Vec<ProposeResult>, WorkflowError>

Execute the queue step: validate, safety-check, route or execute.

When approval_mode is true, replies are queued for human review. When false, replies are executed immediately via toolkit.

All X API writes go through toolkit::write::reply_to_tweet.