atgc repo collaborators lists a repo's collaborators but can't change them. A collaborator is an org.tangled.graph.collaborator record on the repo's knot, keyed by the collaborator's DID, written with service auth from the repo's owner.
Add atgc repo collaborators add <handle|did> and remove <handle|did>, with --dry-run and --json. Refuse up front when the acting account isn't the owner.
This needs the login to request knot record writes for this collection, which it doesn't yet. That scope change needs approval.
Resolved by #1457, now merged.