Format the Markdown with prettier in make fmt and make fmt-check (closes #215)

make fmt and make fmt-check covered only Go, so Markdown formatting was
checked by eye. prettier, pinned in package.json and yarn.lock beside
ESLint and installed by the same js-deps stage, now formats every
Markdown file with the settings in .prettierrc (4-space tabs, prose
wrapped). It runs only in Docker: make fmt writes the formatted files
back from the markdown-output stage, and make fmt-check and the image
build run the markdown-check stage. The Dockerfile's lint stage now
runs the gofmt check itself, since make fmt-check needs a docker daemon.

Model: opus-5-5

script/bootstrap

@@ -3,8 +3,8 @@
 # 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). golangci-lint, node and ESLint are deliberately not
-# installed: linting runs only in docker, via script/lint.
+# make, or go). golangci-lint, node, ESLint and prettier are deliberately
+# not installed: they run only in docker, via script/lint and script/fmt.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -60,9 +60,10 @@ main() {
     if missing go; then pkg_install go golang go go; fi
 
     # Not installed here: docker is platform-specific and out of scope for a
-    # package-manager bootstrap, but script/lint and script/css need it.
+    # package-manager bootstrap, but script/lint, script/fmt and script/css
+    # need it.
     if missing docker; then
-        echo "bootstrap: docker not found; script/lint and script/css require it" >&2
+        echo "bootstrap: docker not found; script/lint, script/fmt and script/css require it" >&2
     fi
 
     go mod download