Function prepare_task 

pub async fn prepare_task(
    task_id: &str,
    agent: Option<&str>,
    model: Option<&str>,
    backend: Option<&dyn ExternalBackend>,
    repo: &str,
    project_dir: &Path,
    attempts: u32,
    store: &Option<Arc<TaskStore>>,
) -> Result<TaskInitResult>

Prepare the full task: set up worktree, build context and prompts, create invocation.