Check Go formatting with gofumpt, as make fmt writes it (closes #70)

The Go format check ran plain gofmt, which accepts code that gofumpt,
the formatter script/fmt runs, rewrites; and the two covered different
files. Both now go through script/gofumpt, which runs one pinned
gofumpt version over every Go file in --write or --check mode, as
script/prettier does for Markdown. It replaces script/fmt-check-go;
make fmt-check-go and the Docker lint stage call it. go run builds the
pinned version, so nothing has to install gofumpt, and the check fails
when gofumpt cannot run instead of passing. Generated mfer/mf.pb.go
passes: gofumpt holds generated files to gofmt's rules. The check is
not redundant: .golangci.yml enables no golangci-lint formatters.

Model: opus-5-5

Dockerfile

@@ -9,7 +9,8 @@ RUN go mod download
 COPY . .
 
 # Go half of fmt-check only: this image has no node, so no prettier. The
-# markdown half runs in the mdfmt stage below.
+# markdown half runs in the mdfmt stage below. The image has no gofumpt
+# either; script/gofumpt builds the version it pins with `go run`.
 RUN make fmt-check-go
 # The linter directly, not `make lint`: script/lint builds this stage, and
 # there is no docker inside this build.