make fmt and make fmt-check cover Markdown with prettier (closes #39)

script/fmt now runs go fmt and then prettier --write on every Markdown
file; script/fmt-check runs the gofmt check and then prettier --check.
prettier is pinned in package.json and yarn.lock and configured in
.prettierrc with four-space indents and prose wrapped at 80 columns,
all copied from the sneak/prompts templates. After Go and the Go
modules, script/bootstrap uses an installed node or else a pinned one
through a hash-verified nvm archive, then an installed yarn or else a
pinned one, then the yarn dependencies. README.md is reformatted by
make fmt, and its Entrypoints section says what fmt, fmt-check and
bootstrap now do.

Model: opus-5-5

yarn.lock

@@ -0,0 +1,8 @@
+# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
+# yarn lockfile v1
+
+
+prettier@3.8.1:
+  version "3.8.1"
+  resolved "https://registry.yarnpkg.com/prettier/-/prettier-3.8.1.tgz#edf48977cf991558f4fcbd8a3ba6015ba2a3a173"
+  integrity sha512-UOnG6LftzbdaHZcKoPFtOcCKztrQ57WkHDeRD9t/PTQtmT0NHSeWWepj6pS0z/N7+08BHFDQVUrfmfMRcZwbMg==