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 now installs Node and Yarn the way the one in
sneak/prompts does, then prettier; 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.

Model: opus-5-5

.prettierrc

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