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/cibuild

@@ -3,17 +3,17 @@
 # push.
 #
 # The Dockerfile runs the gates individually as build steps, not the
-# make check aggregate: the lint stage runs make fmt-check,
+# make check aggregate: the lint stage runs the gofmt check,
 # script/verify-lint-image-pin, golangci-lint config verify and
-# golangci-lint run; the build stage, dropped to an unprivileged user,
-# runs make test and make fmt-check. Neither make lint nor make check
-# appears, because both reach script/lint, which is itself a docker
-# build, and a docker build cannot run inside one. Lint is not skipped
-# by that — the linter is invoked directly in the lint stage, and the
-# build stage's COPY --from=lint makes that stage a prerequisite, so
-# BuildKit must finish it first. Between the two stages everything
-# make check would run has run, which is why a successful build here
-# implies the repo is green.
+# golangci-lint run; the markdown stage runs the prettier check; the
+# build stage, dropped to an unprivileged user, runs make test. None of
+# make lint, make fmt-check or make check appears, because each runs
+# docker, and docker cannot run inside a docker build. Nothing is
+# skipped by that — the linter, gofmt and prettier are invoked directly
+# in their stages, and the build stage's COPY --from lines make those
+# stages prerequisites, so BuildKit must finish them first. Between the
+# three stages everything make check would run has run, which is why a
+# successful build here implies the repo is green.
 #
 # That implication holds only because of CHECK_EPOCH. A COPY layer is
 # invalidated only by changed content, and a rebuild of an unchanged