add a Makefile or justfile with a radicle publish recipe

No build tooling in the repo. Add targets for build, run, fmt, vet, test, and a combined check, plus a publish recipe for Radicle (rad sync --announce, and maybe a tag-and-broadcast target like grrs's publish-tag).

Pick Makefile (like intake) or justfile (like grrs) — whichever we want to standardize on.