Congratulations! This looks amazing. I think I am the target audience and I cannot wait to try this.
One very important thing for my use case is being able to run steps in parallel distributed across multiple agents. Is it capable of this?
Yes :) We rely entirely on buildkit (https://github.com/moby/buildkit) to run the pipelines (`dagger` itself acts as a "compiler" and buildkit client). So anything you can do with buildkit, works out of the box with Dagger. That includes clustering.
There is a lot of ongoing work in that area in the buildkit ecosystem. For example Netflix has a distributed buildkit farm, which I believe relies on containerd storage backends to share cache data. There are also integrations to run buildkit nodes on a kubernetes cluster.
Dagger itself is obviously quite new, but buildkit itself is very mature, thanks to its integration with 'docker build'.