Stop broad objective completion rejection loops

[nathanwhit]

Summary

• ignore review/evaluator gate worker results when checking for unpublished implementation candidates before finishing broad objectives
• add a repeated completion-rejection brake so backend guards cannot keep invoking the replanner forever on the same blocked finish decision
• cover both the stale code-review-gate candidate diff case and the repeated broad-objective rejection loop with tests

Validation

• go test ./internal/orchestrator -run 'TestService(AllowsBroadObjectiveFinishWithReviewGateCandidateDiff|PausesRepeatedBroadObjectiveCompletionRejection|RejectsBroadObjectiveFinishWithUnpublishedCandidate|RejectsBroadObjectiveCompletionWithIncompleteWorkPlan)$'
• go test ./internal/orchestrator ./internal/eventstore
• go test ./internal/orchestrator -run 'TestService(AllowsBroadObjectiveFinishWithReviewGateCandidateDiff|PausesRepeatedBroadObjectiveCompletionRejection)$' -count=20