Why: $queryStats cannot tell server-owned work (resharding, chunk migration) from user operations, so internal shapes show up in Query Shape Insights. Inferring this from session or client state gives false positives, for example user commands that mongos forwards to shards. An explicit origin bit, set where server-owned work starts, is precise. Its only failure mode is a false negative, which a one-line stamp fixes.

What:
* An isInternalOperation opCtx decoration.
* Egress forwards it only when featureFlagInternalOperationOrigin (FCV 9.1) is enabled, so mixed-version clusters never see an unknown field.
* Ingress honors it only from internally authorized clients, so external clients cannot spoof it.
* Nothing sets the flag yet.