Function absorb_scope 

pub fn absorb_scope(
    workspace: &Path,
    db: &Database,
    id: &str,
) -> Result<AbsorbReport>

Absorb a SubBrain into MainBrain: reassign nodes + drop scope def.

Order: DB reassignment first (idempotent), then manifest update so a crash mid-way never leaves a live SubBrain def with nodes already on main.