Create a link / parent edge between two tickets
Inbox
Create a link / parent edge between two tickets
Stamps a link (symmetric, reference) or parent (asymmetric, epic→sub-ticket) edge from :id (source) to target_id. For link the controller canonical-orders the pair (smaller id wins parent slot) so dedupe is automatic. 404 on missing source/target; 422 on self-link; 422 on duplicate edge.
POST
Create a link / parent edge between two tickets