Function close 

pub fn close(
    mana_dir: &Path,
    id: &str,
    opts: CloseOpts,
) -> Result<CloseOutcome, Error>

Close a single unit — the full lifecycle.

Steps: pre-close hook → verify → worktree merge → feature gate → mark closed → archive → post-close cascade → auto-close parents → rebuild index.

Does NOT handle TTY confirmation for feature units — if the unit is a feature, returns CloseOutcome::FeatureRequiresHuman and the caller decides.