Format Markdown with prettier in make fmt (closes #43)

script/fmt and script/fmt-check now run prettier on every Markdown file
after their Go steps, as the standard scripts from sneak/prompts at
dd4027b do. script/bootstrap gains the standard node, nvm and yarn
install next to its goimports install, and the last Dockerfile stage
copies package.json and yarn.lock before running it. .prettierrc,
.prettierignore and yarn.lock are the standard copies. package.json is
the standard one without its licence field and marked private, because
this repo's licence is the WTFPL. .dockerignore keeps the log of a failed
yarn install out of the build context. README.md and TODO.md are
reformatted once with it; REPO_POLICIES.md already matched.

Model: opus-5-5

Dockerfile

@@ -31,6 +31,6 @@ COPY --from=lint /src/go.sum /dev/null
 COPY --from=test /src/go.sum /dev/null
 WORKDIR /src
 COPY script/ script/
-COPY go.mod go.sum ./
+COPY go.mod go.sum package.json yarn.lock ./
 RUN script/bootstrap
 COPY . .