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/cibuild

@@ -0,0 +1,14 @@
+#!/bin/sh
+# script/cibuild: run the CI build. The Dockerfile runs `make check`,
+# so a successful build implies all checks pass. The Gitea workflow
+# runs this on push.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    docker build .
+}
+
+main "$@"