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/bootstrap

@@ -3,10 +3,12 @@
 # this repo. Idempotent: every install is guarded by a check so already
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes nothing is present (not git,
-# make, or go). The linter is NOT installed: golangci-lint runs via
-# docker only (script/lint), pinned by image digest, so the only lint
+# make, or go). Neither the linter nor the Markdown formatter is
+# installed: golangci-lint (script/lint) and prettier (script/fmt,
+# script/fmt-check) run via docker only, pinned by hash, so their only
 # prerequisite is a working docker — which is warned about, not
-# installed, because everything except linting works without it.
+# installed, because everything except linting and formatting works
+# without it.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -71,15 +73,15 @@ main() {
     if missing make; then pkg_install gnumake make make make; fi
     if missing go; then pkg_install go golang go go; fi
 
-    # Linting runs via docker only (script/lint), so docker is a lint
-    # prerequisite rather than something bootstrap installs. Warn, do
-    # not fail: everything except `make lint` — and, through it,
-    # `make check`, `make docker` and the pre-commit hook — works
-    # without it.
+    # Linting and Markdown formatting run via docker only, so docker is
+    # their prerequisite rather than something bootstrap installs. Warn,
+    # do not fail: everything except `make lint`, `make fmt` and
+    # `make fmt-check` — and, through them, `make check`, `make docker`
+    # and the pre-commit hook — works without it.
     if missing docker; then
-        echo "bootstrap: WARNING: docker not found; make lint, make check" >&2
-        echo "bootstrap: and make docker require it. Install docker to" >&2
-        echo "bootstrap: run the linter." >&2
+        echo "bootstrap: WARNING: docker not found; make lint, make fmt," >&2
+        echo "bootstrap: make fmt-check, make check and make docker" >&2
+        echo "bootstrap: require it." >&2
     fi
 
     go mod download