fmt: format and check Markdown with prettier in a container (closes #119)

make fmt and make fmt-check now cover every Markdown file with prettier
(4-space tabs, proseWrap always), as template-app-go does: prettier is
pinned by package.json and yarn.lock and runs in a docker build on a
digest-pinned node image, never on the host. The check is forced to run
with --no-cache-filter, as script/lint is.

script/fmt-check is split into a Go half and a Markdown half because the
Dockerfile lint stage cannot run docker: that stage now runs the Go half
and script/cibuild runs the Markdown half after the build. *.md leaves
.dockerignore so documents reach the build context.

README.md, TESTING.md and TODO.md are reformatted by make fmt; apart from
the README Entrypoints entries, its make fmt line under Building and the
TODO entry, that diff is mechanical.

Model: opus-5-5

script/fmt-check

@@ -1,27 +1,14 @@
 #!/bin/sh
 # script/fmt-check: check formatting (read-only). Same tools and scope
-# as script/fmt, but fails instead of writing.
+# as script/fmt, but fails instead of writing: the Go on the host, the
+# markdown with prettier in a container.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
-
-# goimports v0.42.0, 2026-08-07. Must match script/fmt.
-GOIMPORTS_REF="golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
 
 main() {
-    cd "$ROOT"
-    files="$(gofmt -s -l .)"
-    if [ -n "$files" ]; then
-        echo "gofmt: files not formatted:" >&2
-        echo "$files" >&2
-        exit 1
-    fi
-    files="$(go run "$GOIMPORTS_REF" -l .)"
-    if [ -n "$files" ]; then
-        echo "goimports: files not formatted:" >&2
-        echo "$files" >&2
-        exit 1
-    fi
+    "$SCRIPT_DIR/fmt-check-go"
+    "$SCRIPT_DIR/fmt-check-markdown"
 }
 
 main "$@"