Release v1.2.1.0

This release does not include changes for trident end users.

It adds two new subcommands for (public) archive management, but they are only relevant from a developer's perspective: chronicle creates or updates a dedicated .yml file to document version iterations of Poseidon packages in a Git-managed archive, and timetravel recovers package iterations based on this file to (re)construct said archive from the source repository.

Just as serve, both new subcommands will be omitted in the command line help.