Restore Markdown formatting in fmt/fmt-check and pin prettier (closes #19)

script/fmt and script/fmt-check now run gofmt for Go and prettier for
Markdown; fmt-check reports each independently. prettier is pinned at
3.8.1 by package.json/yarn.lock (integrity hash); .prettierrc and
.prettierignore are the house settings copied from the prompts repo.
script/bootstrap installs node and yarn from the host package manager
and runs `yarn install --frozen-lockfile`.

The Markdown check runs in CI via the Dockerfile build stage, where
bootstrap provides prettier; it is removed from the lint stage because
the golangci-lint image has no node. JS manifests are copied before
bootstrap so the yarn layer caches, and node_modules is dockerignored.

node is an unpinned host runtime like git/make/go: nvm's glibc node does
not run on the musl/Alpine build image, so the canonical nvm route is
not usable here; prettier is the hash-pinned formatter. The wholesale
Markdown reformat follows in the next commit.

Model: opus-4-8

.prettierrc

@@ -0,0 +1,4 @@
+{
+    "tabWidth": 4,
+    "proseWrap": "always"
+}