Merge ticket :id (source) into target_id (Zendesk parity)
Inbox
Merge ticket :id (source) into target_id (Zendesk parity)
Closes :id as resolved, re-parents every comment onto the target ticket, and stamps a merge edge (parent=target, child=:id). Idempotent on duplicate-merge: the second call returns the existing edge as 200. 404 on missing source or target; 422 on self-merge, source-already-merged, or target-already-merged.
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Merge ticket :id (source) into target_id (Zendesk parity)