script/tidy runs `go mod tidy` in a new tidy stage of the Dockerfile,
on the test phase's Go image, and writes go.mod and go.sum back into the
working tree with `docker build --output`. The test phase now runs
`go mod tidy -diff` before the tests and fails naming `make tidy`; it
comes first because a missing go.sum line otherwise fails the tests with
Go's own message.
Judgement call: script/tidy's build has no tag, unlike the other builds
in script/: it makes no image.
Model: opus-5-5
