Live reload is feature that would be nice to have. But for the moment you can have a quick workaround with a single line of Python:

python3 -m http.server 8080

Of course you can add this again to your Makefile:

build-dev: assets
	hagen -b http://localhost:8080 -D

run: build-dev
	cd output && python3 -m http.server 8080

You you can run make run to build and run the server. And make build-dev to rebuild the content.

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