Vendor pinned prettier/format toolchain from prompts scaffold (closes #203)

Replace the unpinned `npx prettier --tab-width 4` in `script/fmt` with the
canonical toolchain vendored from `sneak/prompts`: `.prettierrc` (tabWidth 4,
proseWrap always), pinned `package.json` + `yarn.lock` (prettier 3.8.1), and a
`.prettierignore` that unions the scaffold entries with the repo's `*.min.js`
line. `script/bootstrap` now installs a pinned node/yarn via a hash-verified
nvm release archive (never curl-pipe-sh), so prettier stops being an unpinned
host tool. `script/fmt` runs the pinned prettier via `.prettierrc` over
`static/js/*.js` and `**/*.md`, keeping gofmt/goimports. All existing markdown
is reflowed to house style; `static/js/alpine.min.js` stays byte-identical.

Model: opus-4-8

script/bootstrap

@@ -5,8 +5,12 @@
 # or apk (detected in that order); assumes NOTHING is present (not git,
 # make, or go). goimports is installed with `go install` at a pinned
 # version (integrity via the Go module checksum database) into
-# /usr/local/bin so it is on PATH. The linter is not installed here: it
-# runs only in Docker via script/lint, so docker is its sole prerequisite.
+# /usr/local/bin so it is on PATH. Node is used directly if installed;
+# otherwise it is installed at a pinned version via nvm (installing nvm
+# itself first, from a hash-verified release archive, never curl | sh),
+# then the pinned prettier from yarn.lock. The linter is not installed
+# here: it runs only in Docker via script/lint, so docker is its sole
+# prerequisite.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -15,6 +19,12 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 # golang.org/x/tools goimports, 2026-08-13. v0.49.0 requires Go 1.25 (matches
 # go.mod); v0.50.0 needs Go 1.26. Integrity via the Go module checksum database.
 GOIMPORTS_VERSION="v0.49.0"
+# Node/yarn toolchain, 2026-07-06.
+NODE_VERSION="22.17.0"
+NVM_VERSION="0.40.3"
+# sha256 of https://github.com/nvm-sh/nvm/archive/refs/tags/v0.40.3.tar.gz
+NVM_SHA256="5f4d6aaa04a177dc93c985e31dbc411ab6b8c6e1e21d8015dbc1372625fcd1d0"
+YARN_VERSION="1.22.22"
 
 PKGMGR=""
 SUDO=""
@@ -56,6 +66,21 @@ missing() {
     ! command -v "$1" >/dev/null 2>&1
 }
 
+# verify_sha256 <file> <expected-hash>
+verify_sha256() {
+    if command -v sha256sum >/dev/null 2>&1; then
+        actual="$(sha256sum "$1" | cut -d' ' -f1)"
+    else
+        actual="$(shasum -a 256 "$1" | cut -d' ' -f1)"
+    fi
+    if [ "$actual" != "$2" ]; then
+        echo "bootstrap: sha256 mismatch for $1" >&2
+        echo "  expected: $2" >&2
+        echo "  actual:   $actual" >&2
+        exit 1
+    fi
+}
+
 # goimports is not packaged uniformly across nix/apt/brew/apk, so install it
 # with `go install` at a pinned version and place the binary in /usr/local/bin
 # so it is on PATH regardless of shell config. Requires go, which main
@@ -69,6 +94,54 @@ ensure_goimports() {
     rm -rf "$tmp"
 }
 
+# nvm is a bash script; run a command in a bash with nvm loaded
+nvm_sh() {
+    bash -c ". \"\$HOME/.nvm/nvm.sh\" && $*"
+}
+
+ensure_nvm() {
+    [ -s "$HOME/.nvm/nvm.sh" ] && return 0
+    # nvm prerequisites; nvm itself requires bash
+    if missing bash; then pkg_install bash bash bash bash; fi
+    if missing curl; then pkg_install curl curl curl curl; fi
+    if missing git; then pkg_install git git git git; fi
+    tmp="$(mktemp -d)"
+    curl -fsSL -o "$tmp/nvm.tar.gz" \
+        "https://github.com/nvm-sh/nvm/archive/refs/tags/v${NVM_VERSION}.tar.gz"
+    verify_sha256 "$tmp/nvm.tar.gz" "$NVM_SHA256"
+    mkdir -p "$HOME/.nvm"
+    tar -xzf "$tmp/nvm.tar.gz" -C "$HOME/.nvm" --strip-components=1
+    rm -rf "$tmp"
+}
+
+ensure_node() {
+    if ! missing node; then return 0; fi
+    ensure_nvm
+    nvm_sh "nvm install $NODE_VERSION"
+}
+
+ensure_yarn() {
+    if ! missing yarn; then return 0; fi
+    if ! missing corepack; then
+        corepack enable
+        corepack prepare "yarn@$YARN_VERSION" --activate
+    elif [ -s "$HOME/.nvm/nvm.sh" ]; then
+        nvm_sh "nvm use $NODE_VERSION >/dev/null && corepack enable && \
+            corepack prepare yarn@$YARN_VERSION --activate"
+    else
+        npm install -g "yarn@$YARN_VERSION"
+    fi
+}
+
+install_js_deps() {
+    if missing yarn && [ -s "$HOME/.nvm/nvm.sh" ]; then
+        nvm_sh "nvm use $NODE_VERSION >/dev/null && cd \"$ROOT\" && \
+            yarn install --frozen-lockfile"
+    else
+        yarn install --frozen-lockfile
+    fi
+}
+
 main() {
     cd "$ROOT"
 
@@ -80,6 +153,11 @@ main() {
     if missing go; then pkg_install go golang go go; fi
     ensure_goimports
 
+    # Node toolchain and pinned prettier
+    ensure_node
+    ensure_yarn
+    install_js_deps
+
     # The linter runs only in Docker (script/lint). Warn, don't fail: the
     # rest of the repo works without it.
     if missing docker; then