Add scripts-to-rule-them-all scaffold (closes #4)

Adopt the Scripts to Rule Them All standard for this Hugo site:

- script/ POSIX-sh entrypoints (bootstrap, setup, projectname, test,
  lint, fmt, fmt-check, check, docker, cibuild, precommit,
  install-precommit). The correctness check (test/lint) is a clean
  `hugo --minify` production build; fmt/fmt-check run prettier over the
  repo's own top-level markdown only, leaving content/ untouched.
- Makefile targets reduced to thin shims that call script/NAME, plus a
  convenience serve target for `hugo server`.
- Dockerfile on a sha256-pinned alpine base that installs deps via
  script/bootstrap and runs `make check`, so the image build fails on
  any formatting or Hugo build error; .dockerignore added.
- .gitea/workflows/check.yml runs script/cibuild on push.
- README Entrypoints section documenting the scripts.

script/fmt-check

@@ -0,0 +1,17 @@
+#!/bin/sh
+# script/fmt-check: check the formatting of the repo's own top-level
+# markdown docs (read-only). Same scope as script/fmt, but fails
+# instead of writing.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+PRETTIER_VERSION="3.4.2"
+
+main() {
+    cd "$ROOT"
+    npx --yes "prettier@${PRETTIER_VERSION}" --check \
+        '*.md' --tab-width 4 --prose-wrap always
+}
+
+main "$@"