Format the markdown with prettier in script/fmt and script/fmt-check (closes #100)

script/fmt and script/fmt-check run prettier 3.8.1 on the markdown after
gofmt, with the same yarn helper and arguments as the copies in
sneak/prompts. prettier is pinned in package.json and yarn.lock;
.prettierrc sets four-space tabs and proseWrap always, and
.prettierignore keeps prettier off REPO_POLICIES.md and vendor/. Plain
script/bootstrap installs Node and Yarn the way the one in sneak/prompts
does; with --cgo it does not, as the Dockerfile stages that pass it
format nothing. The HTML templates stay out: prettier cannot parse a Go
template action inside a tag. This commit also holds the reflow that
make fmt then produced (lines rewrapped, bullets as dashes, no word
changed), which the PR kept as a separate commit for review.

Model: opus-5-5

script/fmt-check

@@ -5,6 +5,25 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
+# Must match the pin in script/bootstrap.
+NODE_VERSION="22.17.0"
+
+# script/bootstrap installs node and yarn under nvm and leaves neither
+# on the PATH of the shell that called it, so resolve the pinned
+# toolchain here the way bootstrap's own install step does. nvm is a
+# bash script, hence the subshell.
+run_yarn() {
+    if command -v yarn >/dev/null 2>&1; then
+        exec yarn "$@"
+    fi
+    if [ ! -s "$HOME/.nvm/nvm.sh" ]; then
+        echo "fmt-check: no yarn; run script/bootstrap first" >&2
+        exit 1
+    fi
+    exec bash -c '. "$HOME/.nvm/nvm.sh" && nvm use "$1" >/dev/null &&
+        shift && exec yarn "$@"' bash "$NODE_VERSION" "$@"
+}
+
 main() {
     cd "$ROOT"
     echo "Checking formatting..."
@@ -13,6 +32,7 @@ main() {
         gofmt -l . | grep -v '^vendor/'
         exit 1
     fi
+    run_yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
 }
 
 main "$@"