While Hagen can copy assets, it currently does not process any SCSS, or minimize CSS/Javascript.

One of the reasons for that is, that other tools can do this quite well, and so what is the benefit of replicating that.

With a simple Makefile, you easily trigger and full blown process. For example, this web page processes SCSS simply by:

build/assets/bootstrap/bootstrap.min.css: scss/custom.scss node_modules/bootstrap/scss/bootstrap.scss package.json postcss.config.js
	yarn css:compile
	yarn css:prefix
	yarn css:minify

You can see the full example here: /website/Makefile.