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

package.json

@@ -0,0 +1,5 @@
+{
+  "devDependencies": {
+    "prettier": "3.8.1"
+  }
+}