Format the Markdown with prettier in make fmt and make fmt-check (closes #215)

`make fmt` formatted only Go, so the org's Markdown settings were unenforced and Markdown was wrapped by hand. prettier, pinned in `package.json` and `yarn.lock` beside ESLint, now formats the Markdown with `.prettierrc` (4-space tabs, `proseWrap: always`). It runs only in Docker: `make fmt` writes the formatted files back without a bind mount, and `make fmt-check`, `make check` and the image build fail on unformatted Markdown. The image build's lint stage now runs the Go format check directly and no longer installs `make`. `README.md` and `TODO.md` are reformatted with no word changed: the README reflows from 72 to 80 columns.

Model: opus-5-5

script/bootstrap

@@ -3,8 +3,8 @@
 # this repo. Idempotent: every install is guarded by a check so already
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes NOTHING is present (not git,
-# make, or go). golangci-lint, node and ESLint are deliberately not
-# installed: linting runs only in docker, via script/lint.
+# make, or go). golangci-lint, node, ESLint and prettier are deliberately
+# not installed: they run only in docker, via script/lint and script/fmt.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -60,9 +60,10 @@ main() {
     if missing go; then pkg_install go golang go go; fi
 
     # Not installed here: docker is platform-specific and out of scope for a
-    # package-manager bootstrap, but script/lint and script/css need it.
+    # package-manager bootstrap, but script/lint, script/fmt and script/css
+    # need it.
     if missing docker; then
-        echo "bootstrap: docker not found; script/lint and script/css require it" >&2
+        echo "bootstrap: docker not found; script/lint, script/fmt and script/css require it" >&2
     fi
 
     go mod download