Format Markdown with prettier in make fmt and fmt-check (closes #23)

`make fmt` now runs prettier over the Markdown files after the Go
formatters, and `make fmt-check` fails when prettier would change one.
`package.json`, `yarn.lock`, `.prettierrc` and `.prettierignore` are the
`sneak/prompts` copies at `dd4027b`, and both scripts find yarn the way
that repo's scripts do. `script/bootstrap` installs prettier with
`yarn install --frozen-lockfile`. When the yarn those scripts would run
is not 1.22.22, it installs yarn through corepack under node 22.17.0:
the node on `PATH` if it has that version, otherwise one installed
through nvm from a hash-checked archive. `README.md` and `TODO.md` are
reformatted once.

Model: opus-5-5

package.json

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