No entrypoint could change yarn.lock or go.mod: script/bootstrap
installs with --frozen-lockfile, so adding a package meant running
yarn by hand. make add-dependency PACKAGE=<name>@<version> shims to
the new script/add-dependency: yarn add --dev, then yarn install
--frozen-lockfile. make tidy shims to the new script/tidy, go mod
tidy in backend/; a Go module is added by importing it, or moved by
editing its require line, then make tidy. script/bootstrap is
unchanged.
Model: opus-5-5
