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

package.json

@@ -0,0 +1,5 @@
+{
+  "devDependencies": {
+    "prettier": "3.8.1"
+  }
+}