Feature Request: Add option to use JJ VCS system to enable use on existing Git/Piper/Mercurial/etc. projects.

Jujustu (https://github.com/jj-vcs/jj) is an open source VCS that uses other VCS systems such as git or piper as its backend.

We should add the option for conductor to use JJ as an alternative to git if the user so desires. This creates a unified API for interacting with a variety of different VCS types.

The main benefit is that this would enable conductor to work on existing brownfield projects that use a JJ supported VCS systems other than git such as Piper/CitC projects without manually adding integrations for each of them.

My thoughts are that for greenfield projects we can ask the user if they want to use git or JJ (defaulting to git to not disrupt existing workflows). For brownfield projects we will use git for detected git projects, or JJ if we detect a supported VCS other than git. We can then store this preference in the tech stack document to store the desired workflow.