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

script/fmt

@@ -25,6 +25,13 @@ run_yarn() {
 
 main() {
     cd "$ROOT"
+    # Without the repo's own prettier, yarn runs whatever prettier is on
+    # the PATH, which may be a version that formats files differently.
+    if [ ! -x node_modules/.bin/prettier ]; then
+        echo "fmt: this repo's prettier is not installed in node_modules/," \
+            "so run script/bootstrap first." >&2
+        exit 1
+    fi
     echo "Formatting..."
     run_yarn run fmt
 }