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>

package.json

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