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 installs Node and Yarn the way the one in sneak/prompts
does; 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. This commit also holds the reflow that
make fmt then produced (lines rewrapped, bullets as dashes, no word
changed), which the PR kept as a separate commit for review.

Model: opus-5-5

package.json

@@ -0,0 +1,6 @@
+{
+  "license": "GPL-3.0",
+  "devDependencies": {
+    "prettier": "3.8.1"
+  }
+}