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

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