Fence cron activation on callee readiness

Cross-service cron releases need an explicit readiness fence between the caller and the endpoint deployment.

Applicability: one-shot or scheduled services that immediately call another service during deployment. A successful build of the caller does not prove the callee is already serving the compatible revision. Keep the schedule disabled until the endpoint release is live, run one bounded manual invocation, and enable cadence only after that succeeds. An internal work budget should remain shorter than the transport timeout so the endpoint can return an honest partial result instead of being killed.

Limits: this does not replace idempotency, bounded batches, or provider/database deadlines. A successful partial response proves operational execution, not complete data coverage.