Function archive_task 

pub fn archive_task(
    base: &Path,
    tasks: &HashMap<String, Task>,
    id_or_prefix: &str,
) -> Result<Vec<String>>

Archive a single task (and its cascade) identified by id_or_prefix.

Cascade rules:

• If the task is an epic, its Done/Cancelled children are also archived (children that are not settled block the archive if they themselves would block archiving, but epic children are just included when settled).
• For any archived task, its settled depends_on tasks that are no longer depended on by any active task are NOT automatically cascaded here — the caller may sweep afterwards with archive_all.

On failure returns Error::NotArchivable listing active dependents.