To make it short; yes you can contribute! The management system uses AGPL-3.0-only, to make sure the code stay open and free, but other subsystems may use other licenses. In particular the models use non-standard licenses, some of which may seem alien.
The code is provided as a share of ideas, not as a private repo, but neither as a fully public repo. It is for testing ideas, not for production of industry bots. The code as such is a brainchild that has emerged over several years, parts of it is from my cand.scient. thesis back in the 1990s, and I strongly doubt it will be finalized any time soonish. If you provide any code, please keep that in mind.
If you want to share your contributions, the way to do it is to create your own clone of the repository and make pull requests from branches you create in your own repository. Please respect use of linters, and you should also check your own code.
Note that I might choose at some point to change the license for the whole or parts of the repository.