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>

script/fmt-check

@@ -1,10 +1,30 @@
 #!/bin/sh
 # script/fmt-check: check formatting (read-only). Same scope as
-# script/fmt, but fails instead of writing.
+# script/fmt, the Go code and every Markdown file, but fails instead of
+# writing.
 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 "$ROOT"
     unformatted="$(gofmt -l .)"
@@ -13,6 +33,8 @@ main() {
         echo "$unformatted" >&2
         exit 1
     fi
+    # run_yarn replaces this shell, so it stays the last step.
+    run_yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
 }
 
 main "$@"