Format the Markdown with prettier in make fmt and make fmt-check (closes #215)

`make fmt` formatted only Go, so the org's Markdown settings were unenforced and Markdown was wrapped by hand. prettier, pinned in `package.json` and `yarn.lock` beside ESLint, now formats the Markdown with `.prettierrc` (4-space tabs, `proseWrap: always`). It runs only in Docker: `make fmt` writes the formatted files back without a bind mount, and `make fmt-check`, `make check` and the image build fail on unformatted Markdown. The image build's lint stage now runs the Go format check directly and no longer installs `make`. `README.md` and `TODO.md` are reformatted with no word changed: the README reflows from 72 to 80 columns.

Model: opus-5-5

script/fmt

@@ -1,5 +1,7 @@
 #!/bin/sh
-# script/fmt: format all files (writes).
+# script/fmt: format all files (writes): the Go code with gofmt and
+# goimports, the Markdown with prettier. prettier is never installed
+# locally: it runs in docker, in the Dockerfile's Markdown stages.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -10,6 +12,7 @@ main() {
     if command -v goimports >/dev/null 2>&1; then
         goimports -w .
     fi
+    docker build --target markdown-output --output type=local,dest=. .
 }
 
 main "$@"