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

script/fmt ran prettier with default settings over root-level *.md and
*.json, swallowing every failure with `|| true`, while script/fmt-check
checked gofmt only. The formatter and the gate therefore disagreed
silently: `make fmt` rewrote markdown that `make check` never looked at,
including REPO_POLICIES.md, which is a verbatim copy of an authoritative
upstream document that local tooling must not touch.

Configuration:

- .prettierrc pins the two policy deviations from prettier defaults,
  four-space indents and proseWrap: always. Nothing else.
- .prettierignore excludes REPO_POLICIES.md so no local run can drift it
  from upstream again, plus .golangci.yml (user-owned, and listed even
  though the current file set does not reach it) and node_modules,
  vendor, bin.

One canonical file set:

- New script/prettier takes --write or --check and applies the same
  patterns in both modes, so script/fmt and script/fmt-check cannot
  drift apart by construction. The patterns are repo-wide (**/*.md,
  **/*.json) rather than root-only, so markdown in subdirectories such
  as a future docs/ is covered.
- No `|| true` anywhere, and no --no-error-on-unmatched-pattern: both
  patterns always match tracked files, so an empty match means the glob
  broke and prettier should say so instead of passing vacuously. A
  missing prettier is a hard error naming script/bootstrap, not a
  silent skip.

Pinned prettier:

- package.json/yarn.lock pin prettier 3.9.6; the lockfile carries the
  integrity hash, and --frozen-lockfile enforces it. script/prettier
  prefers node_modules/.bin/prettier and warns on stderr when it has to
  fall back to a PATH prettier of unknown version.
- script/bootstrap now installs node, yarn, and the locked JS deps. Its
  NODE_VERSION and YARN_VERSION pins already existed.

Docker gate:

- The golangci-lint image has no node, so the lint stage runs the new
  script/fmt-check-go (the Go half of fmt-check, extracted) instead of
  the whole thing.
- The markdown half gets its own stage on a digest-pinned node image
  shipping exactly the node and yarn versions bootstrap pins. The
  builder stage takes a COPY --from dependency on it, so BuildKit cannot
  skip it and a markdown violation fails `docker build .` rather than
  being skipped somewhere nobody looks.

Markdown files other than REPO_POLICIES.md are reformatted here for the
first time under the policy settings.

script/prettier

@@ -0,0 +1,69 @@
+#!/bin/sh
+# script/prettier: run prettier over this repo's canonical file set.
+#
+# Takes exactly one mode argument, --write or --check, and applies the
+# same patterns in both modes. script/fmt and script/fmt-check both go
+# through here, so the set of files that get formatted and the set that
+# get verified cannot drift apart.
+#
+# Failures are never swallowed: a missing prettier is an error, not a
+# silent skip. A formatter that quietly does nothing is worse than one
+# that fails loudly.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+usage() {
+    echo "usage: script/prettier --write|--check" >&2
+    exit 2
+}
+
+# Prefer the version pinned by package.json/yarn.lock so that CI and
+# developer machines format identically. Fall back to a prettier on PATH,
+# but say so, because a different version formats differently.
+find_prettier() {
+    if [ -x "$ROOT/node_modules/.bin/prettier" ]; then
+        printf '%s\n' "$ROOT/node_modules/.bin/prettier"
+        return 0
+    fi
+    if command -v prettier >/dev/null 2>&1; then
+        echo "prettier: node_modules/.bin/prettier is absent; using the" \
+            "prettier on PATH, which may be a different version than the" \
+            "one pinned in package.json. Run script/bootstrap to install" \
+            "the pinned version." >&2
+        command -v prettier
+        return 0
+    fi
+    return 1
+}
+
+main() {
+    [ "$#" -eq 1 ] || usage
+    case "$1" in
+        --write | --check) mode="$1" ;;
+        *) usage ;;
+    esac
+
+    cd "$ROOT"
+
+    if ! prettier_bin="$(find_prettier)"; then
+        echo "prettier: not found." >&2
+        echo "  Install it with: script/bootstrap" >&2
+        echo "  (installs the version pinned in package.json/yarn.lock)" >&2
+        exit 1
+    fi
+
+    # Markdown and JSON, repo-wide rather than root-only, so files in
+    # subdirectories (docs/, once it exists) are covered too. Exclusions
+    # live in .prettierignore; REPO_POLICIES.md is excluded there because
+    # it is a verbatim copy of an upstream document.
+    #
+    # --no-error-on-unmatched-pattern is deliberately NOT used: both
+    # patterns always match at least one tracked file (README.md,
+    # package.json), so an empty match means the glob broke, and prettier
+    # erroring out is exactly what we want rather than a vacuous pass.
+    "$prettier_bin" "$mode" "**/*.md"
+    "$prettier_bin" "$mode" "**/*.json"
+}
+
+main "$@"