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, configured in
.prettierrc with four-space indents and prose wrapped at 80 columns,
all copied from the sneak/prompts templates. script/bootstrap installs
node through a hash-verified nvm archive when it is missing, yarn at a
pinned version, and the yarn dependencies. README.md is reformatted by
make fmt, and its Entrypoints section says what fmt, fmt-check and
bootstrap now cover.

Model: opus-5-5

script/fmt-check

@@ -5,6 +5,25 @@ 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-check: 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 "$ROOT"
     if [ -n "$(gofmt -l .)" ]; then
@@ -12,6 +31,7 @@ main() {
         gofmt -l .
         exit 1
     fi
+    run_yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
 }
 
 main "$@"