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

Adds .prettierrc and .prettierignore, a single script/prettier entrypoint shared by fmt and fmt-check so the two cannot drift, and prettier 3.9.6 pinned by yarn.lock integrity hash.

Markdown formatting is now gated in the authoritative Docker build via a new mdfmt stage, since the golangci-lint image has no node. REPO_POLICIES.md is ignored so local tooling cannot drift it from upstream.

Removes the || true that made the previous prettier invocation unable to fail.

script/bootstrap

@@ -130,9 +130,13 @@ main() {
     if missing make; then pkg_install gnumake make make make; fi
 
     # ---- JS / docs repos ----
-    # ensure_node
-    # ensure_yarn
-    # install_js_deps
+    # This is a Go repo, but node and yarn are required anyway: prettier
+    # formats the Markdown and JSON, and script/fmt-check verifies it.
+    # The version is pinned by package.json/yarn.lock, whose integrity
+    # hashes --frozen-lockfile enforces.
+    ensure_node
+    ensure_yarn
+    install_js_deps
 
     # ---- Go repos ----
     if missing go; then pkg_install go golang go go; fi