Function load_assignment 

pub fn load_assignment(
    cwd: &Path,
    unit_id: &str,
) -> Result<WorkerAssignment, Box<dyn Error>>

Load a worker assignment from a mana unit using canonical mana-core APIs.

This replaces the ad hoc markdown-scanning load_mana_unit() that lived in imp-cli. It uses mana_core::api::get_unit() for canonical resolution and mana_core::discovery::find_mana_dir() for .mana/ discovery.