Make fmt-check check what fmt writes (closes #22)

script/fmt writes with gofmt -s and goimports, but script/fmt-check
only ran gofmt -l. A file that gofmt -s would simplify, or whose
imports goimports would regroup, passed make check and then changed
on the next make fmt.

script/fmt-check now runs gofmt -s -l and goimports -l, prints each
file either would change once, and fails. It puts Go's bin directory
on PATH first, as script/fmt does, so it runs the goimports that
script/bootstrap installs. The README line on what make check runs
now names both formatters.

Model: opus-5-5

README.md

@@ -97,7 +97,7 @@ Future improvements under consideration:
 * Issues / PRs: <https://git.eeqj.de/sneak/attrsum/>
 * Code must pass `make check`, which runs the tests and golangci-lint as
   phases of the `Dockerfile` (Docker is required) and checks formatting
-  with `gofmt`.
+  with `gofmt -s` and goimports.
 * No CLA; contributions are under WTFPL v2.
 
 ---

TODO.md

@@ -24,6 +24,10 @@ Author are missing)
 
 # Completed Steps
 
+* 2026-10-06: `script/fmt-check` checks what `script/fmt` writes: it
+    fails and lists the files when `gofmt -s` or goimports would change
+    one, so a file that passes `make check` no longer changes on the
+    next `make fmt`
 * 2026-10-06: canonical files re-vendored from `sneak/prompts` at
     `dd4027b`: `REPO_POLICIES.md` and `.editorconfig` added;
     `.dockerignore`, `.gitignore`, `.golangci.yml` and the workflow

script/fmt-check

@@ -7,10 +7,15 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    files="$(gofmt -l .)"
+    # script/bootstrap installs goimports into Go's bin directory, which
+    # need not be on PATH.
+    gobin="$(go env GOBIN)"
+    [ -n "$gobin" ] || gobin="$(go env GOPATH)/bin"
+    PATH="$gobin:$PATH"
+    files="$(gofmt -s -l .; goimports -l .)"
     if [ -n "$files" ]; then
-        echo "gofmt: files not formatted:" >&2
-        echo "$files" >&2
+        echo "files that make fmt would change:" >&2
+        echo "$files" | sort -u >&2
         exit 1
     fi
 }