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

make fmt and make fmt-check now cover the Markdown files with prettier as well as the Go source: prettier 3.8.1 pinned in package.json and yarn.lock, four-space indents and proseWrap always in .prettierrc, all three copied unchanged from the sneak/prompts templates. script/bootstrap adds pinned node (through nvm from a hash-checked archive when none is installed), yarn and prettier after the pinned Go. README.md is reformatted by make fmt and its Entrypoints section says what each step needs.

Model: opus-5-5

script/fmt-check

@@ -8,6 +8,25 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 # Where script/bootstrap installs Go; it cannot put it on our PATH.
 PATH="$HOME/.local/go/bin:$PATH"
 
+# 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
@@ -15,6 +34,7 @@ main() {
         gofmt -l .
         exit 1
     fi
+    run_yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
 }
 
 main "$@"