I don't use sourcehut, but interpreting what you wrote I'd argue this is an antifeature and would be a dealbreaker for me. CI typically evolves with the underlying code and decoupling that from the code makes it difficult to go backwards. It loses cohesion.
you can put them in the same repository, if that is your thing.
If you put the build files in a .builds/ folder at the root of your repository, they will be run upon each commit. Just like in github or gitlab. You are just not forced into this way of life.
If you prefer, you can store the build files separately, and run them independently of your commits. Moreover, the build files don't need to be associated to any repository, inside or outside sourcehut.
I don't use sourcehut, but interpreting what you wrote I'd argue this is an antifeature and would be a dealbreaker for me. CI typically evolves with the underlying code and decoupling that from the code makes it difficult to go backwards. It loses cohesion.