Format Markdown with prettier in make fmt and make fmt-check (closes #19)

script/fmt and script/fmt-check run prettier over every Markdown file
again, next to gofmt. prettier is pinned by hash through package.json
and yarn.lock, copied from the prompts repo with .prettierrc and
.prettierignore, and is never installed on a host: a new prettier stage
of the Dockerfile installs it into a digest-pinned node image, and both
scripts build that stage and run it with the repository mounted. CI
checks the Markdown in a markdown stage that the build stage waits on.
Because make fmt-check now runs docker, the Dockerfile runs gofmt
directly in its lint stage instead. All Markdown is reformatted.

Model: opus-5-5

script/docker

@@ -4,11 +4,11 @@
 #
 # CHECK_EPOCH is passed for the same reason script/cibuild passes it:
 # without it Docker serves the Dockerfile's gate layers from cache on an
-# unchanged tree and this exits 0 having run neither the lint stage's
-# gates nor the builder stage's test and fmt-check gates. This is the
-# set of gates a developer or reviewer runs by hand, so a cached pass
-# here is the most misleading result the repo can produce. Dependency
-# layers sit above the ARG and stay cached.
+# unchanged tree and this exits 0 having run none of the lint stage's
+# gates, the markdown stage's prettier gate or the builder stage's test
+# gate. This is the set of gates a developer or reviewer runs by hand,
+# so a cached pass here is the most misleading result the repo can
+# produce. Dependency layers sit above the ARG and stay cached.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"