Format Markdown with prettier in make fmt and make fmt-check (closes #7)

script/fmt now also runs prettier over every Markdown file, and
script/fmt-check checks them without writing. prettier is pinned in
package.json and yarn.lock. script/bootstrap keeps its Go and apt
handling and gains the canonical node and yarn install (nvm from a
hash-checked archive, yarn through corepack), at the pinned versions
when they are absent; a node or yarn already installed is used as is.
Both fmt scripts find yarn the canonical way. The new files come from
sneak/prompts at cc440118c876. README.md and TODO.md are rewrapped by
make fmt, with no wording changes.

Deviation: package.json drops the canonical "license": "MIT" line; this
repo is WTFPL.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

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==