Format Markdown with prettier in script/fmt and script/fmt-check

script/fmt and script/fmt-check now run prettier over all Markdown
after the Go step. The prettier config, package.json, yarn.lock and the
node and yarn steps of script/bootstrap are copied from sneak/prompts
with the same pinned versions.

Model: opus-5-5

script/fmt

@@ -4,11 +4,31 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
+# Must match the pin in script/bootstrap.
+NODE_VERSION="22.17.0"
+
+# script/bootstrap installs node and yarn under nvm and leaves neither
+# on the PATH of the shell that called it, so resolve the pinned
+# toolchain here the way bootstrap's own install step does. nvm is a
+# bash script, hence the subshell.
+run_yarn() {
+    if command -v yarn >/dev/null 2>&1; then
+        exec yarn "$@"
+    fi
+    if [ ! -s "$HOME/.nvm/nvm.sh" ]; then
+        echo "fmt: no yarn; run script/bootstrap first" >&2
+        exit 1
+    fi
+    exec bash -c '. "$HOME/.nvm/nvm.sh" && nvm use "$1" >/dev/null &&
+        shift && exec yarn "$@"' bash "$NODE_VERSION" "$@"
+}
+
 main() {
     cd "$ROOT"
     # Where script/bootstrap installs Go when the host has none.
     PATH="$PATH:$ROOT/.tool/go/bin"
     go fmt ./...
+    run_yarn run prettier --write '**/*.md' --tab-width 4 --prose-wrap always
 }
 
 main "$@"