fix: make fmt refuses to run without the repo's own prettier (closes #509)

In a clone where script/bootstrap has not run, node_modules/ is empty and
yarn falls back to whatever prettier is on the PATH, which can format files
differently from the version yarn.lock pins. script/fmt and script/fmt-check
now stop with a message telling the user to run script/bootstrap when
node_modules/.bin/prettier is missing. After bootstrap they behave as
before. The README Entrypoints entries say so.

Model: opus-5-5

README.md

@@ -239,8 +239,12 @@ provide:
   `make check` stays non-mutating. Linting runs only in the container, because
   an ESLint result that depends on whichever ESLint the host happens to have is
   not a result, so docker is required to lint.
-- `script/fmt` — format all files (writes), on the host
-- `script/fmt-check` — check formatting (read-only), on the host
+- `script/fmt` — format all files (writes), on the host, with the `prettier`
+  that `script/bootstrap` installs in `node_modules/`; when that one is missing
+  it fails and says to run `script/bootstrap`, instead of using a `prettier`
+  from the `PATH`
+- `script/fmt-check` — check formatting (read-only), on the host, with the same
+  `prettier` and the same refusal
 - `script/check` — run `script/test`, `script/lint` and `script/fmt-check`
 - `script/check-censored` — assert the competitor name RULES.md bars appears
   nowhere in the working tree or under `dist/` outside its documented