Restore Markdown formatting in fmt/fmt-check and pin prettier (closes #19)

script/fmt and script/fmt-check now run gofmt for Go and prettier for
Markdown; fmt-check reports each independently. prettier is pinned at
3.8.1 by package.json/yarn.lock (integrity hash); .prettierrc and
.prettierignore are the house settings copied from the prompts repo.
script/bootstrap installs node and yarn from the host package manager
and runs `yarn install --frozen-lockfile`.

The Markdown check runs in CI via the Dockerfile build stage, where
bootstrap provides prettier; it is removed from the lint stage because
the golangci-lint image has no node. JS manifests are copied before
bootstrap so the yarn layer caches, and node_modules is dockerignored.

node is an unpinned host runtime like git/make/go: nvm's glibc node does
not run on the musl/Alpine build image, so the canonical nvm route is
not usable here; prettier is the hash-pinned formatter. The wholesale
Markdown reformat follows in the next commit.

Model: opus-4-8

script/fmt

@@ -1,12 +1,23 @@
 #!/bin/sh
-# script/fmt: format all files (writes).
+# script/fmt: format all files (writes). gofmt for Go, prettier for
+# Markdown. prettier is the pinned devDependency in package.json/
+# yarn.lock; script/bootstrap installs it (see run_prettier).
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
+run_prettier() {
+    if ! command -v yarn >/dev/null 2>&1; then
+        echo "fmt: yarn not found; run script/bootstrap first" >&2
+        exit 1
+    fi
+    yarn run prettier "$@"
+}
+
 main() {
     cd "$ROOT"
     gofmt -s -w .
+    run_prettier --write '**/*.md' --tab-width 4 --prose-wrap always
 }
 
 main "$@"