Format and check markdown with prettier in make fmt and fmt-check (closes #110)

script/fmt and script/fmt-check follow the model scripts in the prompts
repo: Go as before, plus prettier over every markdown file with 4-space
tabs and proseWrap always. Prettier is pinned by hash in package.json and
yarn.lock; script/bootstrap now installs node, yarn and prettier. The
Dockerfile lint stage copies node and yarn from a node image pinned by
hash and runs script/bootstrap, so its make fmt-check fails the build on
unformatted markdown. Every markdown file is formatted once; wording is
unchanged (CLAUDE.md's "*" list markers become "-").

Model: opus-5-5

script/fmt

@@ -1,12 +1,33 @@
 #!/bin/sh
-# script/fmt: format all files (writes).
+# script/fmt: format all files (writes): Go with go fmt, markdown with
+# prettier.
 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: 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"
     go fmt ./...
+    run_yarn run prettier --write '**/*.md' --tab-width 4 --prose-wrap always
 }
 
 main "$@"