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

Dockerfile

@@ -1,10 +1,22 @@
+# node and yarn, copied into the lint stage for prettier, which checks the
+# markdown formatting: node of the version script/bootstrap pins, built on
+# Debian as the lint stage's image is.
+# node:22.17.0-bookworm-slim, 2025-07-08
+FROM node@sha256:b04ce4ae4e95b522112c2e5c52f781471a5cbc3b594527bcddedee9bc48c03a0 AS node
+
 # Lint stage — fast feedback on formatting and lint issues
 # golangci/golangci-lint:v2.12.2 (Debian-based), 2026-08-07
 FROM golangci/golangci-lint:v2.12.2@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240 AS lint
 
+COPY --from=node /usr/local/bin/node /usr/local/bin/node
+COPY --from=node /opt/yarn-v1.22.22 /opt/yarn-v1.22.22
+ENV PATH="/opt/yarn-v1.22.22/bin:${PATH}"
+
+# script/bootstrap downloads the Go modules and installs prettier
 WORKDIR /src
-COPY go.mod go.sum ./
-RUN go mod download
+COPY script/ script/
+COPY go.mod go.sum package.json yarn.lock ./
+RUN script/bootstrap
 
 # script/cibuild sets CHECK_EPOCH to the current time, so the RUN steps
 # below run again on each build, an unchanged tree included, while the