< developers downloading CI artifacts and manually signing and uploading them

Hell no. CI needs to be a clean environment, without any human hands in the loop.

Publishing to public registries should require a chain of signatures. CI should refuse to build artifacts from unsigned commits, and CI should attach an additional signature attesting that it built the final artifact based on the original signed commit. Public registries should confirm both the signature on the commit and the signature on the artifact before publishing. Developers without mature CI can optionally use the same signature for both the source commit and the artifact (i.e. to attest to artifacts they built on their laptop). Changes to signatures should require at least 24 hours to apply and longer (72 hours) for highly popular foundation packages.