429, honor Retry-After and use exponential
backoff with jitter.
Retry reads after transient 429 or 5xx failures. For writes, follow the operation’s
retrySafety: idempotent operations may be replayed with the same identity; at-most-once
operations require outcome reconciliation and must not be blindly resubmitted. A
confirmation result with an unknown outcome must be reconciled before any new action.
Shared error examples are in examples/shared/errors.json; operation-specific replay,
conflict, permission, and asynchronous outcomes are linked from the operation examples.