Maybe something fossil-adjacent:

- amber.dev

- quarry.sh

You’ll probably need to play with gTLDs to find something that works.

Can also echo “scm” from fossil’s domain:

- amberscm.dev

Along with useX.com, Xhq.com, etc., patterns.

Of the two you have listed, I’d choose fossilforge, but would vote for an alternative TLD since .io has an expected meaning coming from GitHub.