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

@@ -1,19 +1,65 @@
 #!/bin/sh
-# script/fmt: format all files (writes).
+# script/fmt: format all files (writes). Go with gofmt and goimports on
+# the host, markdown with the prettier pinned by Dockerfile.fmt.
 #
 # goimports runs with `go run` at a pinned commit, never from PATH, so
 # every machine formats with the same version and nothing installs it.
+#
+# The markdown pass is a `docker build --output type=local` rather than a
+# `docker run -v`, so it needs no bind mount and behaves the same against
+# a remote daemon; the formatted documents come back out of the build and
+# are copied over the tree here.
+#
+# Unlike script/fmt-check-markdown this does not bust the cache: it is
+# not a gate, and any edit to a document changes the COPY layer above the
+# prettier step, so a cached result is a result over this exact tree.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# goimports v0.42.0, 2026-08-07. Must match script/fmt-check.
+# goimports v0.42.0, 2026-08-07. Must match script/fmt-check-go.
 GOIMPORTS_REF="golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0"
 
+# Must match the export stage name in Dockerfile.fmt.
+stage=fmt-out
+
+die() {
+    echo "script/fmt: $*" >&2
+    exit 1
+}
+
 main() {
     cd "$ROOT"
     gofmt -s -w .
     go run "$GOIMPORTS_REF" -w .
+
+    tmp="$(mktemp -d "${TMPDIR:-/tmp}/dnswatcher-fmt.XXXXXX")"
+    trap 'rm -rf "$tmp"' EXIT INT TERM
+
+    docker build \
+        --target "$stage" \
+        --output "type=local,dest=$tmp/out" \
+        -f Dockerfile.fmt .
+
+    # An empty export means prettier was handed nothing, which must not
+    # read as "already formatted".
+    (cd "$tmp/out" && find . -type f -name '*.md') |
+        sed 's|^\./||' | LC_ALL=C sort >"$tmp/files"
+
+    [ -s "$tmp/files" ] ||
+        die "the formatting build produced no markdown; the build" \
+            "context reached prettier empty"
+
+    # Copied only where the bytes differ, so an already-formatted tree
+    # keeps its timestamps and says nothing.
+    while IFS= read -r f; do
+        [ -n "$f" ] || continue
+        if [ -f "$f" ] && cmp -s "$tmp/out/$f" "$f"; then
+            continue
+        fi
+        cp "$tmp/out/$f" "$f"
+        echo "prettier: reformatted $f"
+    done <"$tmp/files"
 }
 
 main "$@"