Cosign architecture-specific images too?

Nobody has claimed this yet.

If I `regctl image copy --referrers ghcr.io/cloudnative-pg/postgresql:18.4-standard-trixie private.goharbor.instance.com/cloudnative-pg/image:18.4`, the top level view appears like this with a green checkmark:

but if I click the "view this OCI index's artifact list" I get this:

so the architecture-specific images aren't signed. This means I can't enable the feature in Harbor that prevents downloads of non-cosigned images since the images from this repository aren't signed recursively.

Is there a reason only the top manifest is signed, or would it be possible to add `--recursive` to the `cosign sign` command that produces the release images in this organization?

Search the release configuration that runs the `cosign sign` command for the PostgreSQL images and inspect how multi-architecture indexes are handled. Verify the resulting OCI artifact list in Harbor, including each architecture-specific image, and confirm that the release workflow still completes successfully.

Written by the indexing model from the issue text.