Format and check markdown with prettier in make fmt and fmt-check (closes #110)

script/fmt and script/fmt-check follow the model scripts in the prompts
repo: Go as before, plus prettier over every markdown file with 4-space
tabs and proseWrap always. Prettier is pinned by hash in package.json and
yarn.lock; script/bootstrap now installs node, yarn and prettier. The
Dockerfile lint stage copies node and yarn from a node image pinned by
hash and runs script/bootstrap, so its make fmt-check fails the build on
unformatted markdown. Every markdown file is formatted once; wording is
unchanged (CLAUDE.md's "*" list markers become "-").

Model: opus-5-5

script/bootstrap

@@ -131,9 +131,10 @@ main() {
     if missing make; then pkg_install gnumake make make make; fi
 
     # ---- JS / docs repos ----
-    # ensure_node
-    # ensure_yarn
-    # install_js_deps
+    # prettier, pinned in package.json and yarn.lock, formats the markdown
+    ensure_node
+    ensure_yarn
+    install_js_deps
 
     # ---- Go repos ----
     if missing go; then pkg_install go golang go go; fi