Configure prettier and make fmt-check cover markdown (closes #69)

Adds .prettierrc and .prettierignore, a single script/prettier entrypoint shared by fmt and fmt-check so the two cannot drift, and prettier 3.9.6 pinned by yarn.lock integrity hash.

Markdown formatting is now gated in the authoritative Docker build via a new mdfmt stage, since the golangci-lint image has no node. REPO_POLICIES.md is ignored so local tooling cannot drift it from upstream.

Removes the || true that made the previous prettier invocation unable to fail.

script/fmt-check

@@ -1,28 +1,14 @@
 #!/bin/sh
 # script/fmt-check: check formatting (read-only). Same scope as
-# script/fmt, but fails instead of writing.
+# script/fmt, but fails instead of writing: Go via script/fmt-check-go,
+# Markdown and JSON via script/prettier.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
-
-# Regenerate mfer/mf.pb.go from mfer/mf.proto if it is missing or stale
-# (mirrors the old Makefile prerequisite; the generated file is
-# committed, so this is normally a no-op).
-ensure_pb() {
-    if [ ! -f mfer/mf.pb.go ] ||
-        [ -n "$(find mfer/mf.proto -newer mfer/mf.pb.go 2>/dev/null)" ]; then
-        (cd mfer && go generate .)
-    fi
-}
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
 
 main() {
-    cd "$ROOT"
-    ensure_pb
-    if [ -n "$(gofmt -l .)" ]; then
-        echo "gofmt: files need formatting:" >&2
-        gofmt -l . >&2
-        exit 1
-    fi
+    "$SCRIPT_DIR/fmt-check-go"
+    "$SCRIPT_DIR/prettier" --check
 }
 
 main "$@"