Re-vendor the shared files from sneak/prompts at dd4027b (closes #504)

Fetches the shared files unchanged from sneak/prompts commit dd4027b and
adds .prettierignore; .dockerignore keeps this repository's anchored host
artifacts at its end.

Linting moves into the Dockerfile's lint phase on golangci-lint v2.14.0,
which also runs ESLint, and Dockerfile.lint is gone. Tests move into a test
phase on the Debian Go image. script/lint, test, docker and cibuild are the
model scripts, every docker build in script/ passes --no-cache, and the
.ci-fingerprint barrier is gone. The workflow no longer calls
script/ci-mark-superseded, so it and its tests are removed. make build
passes -trimpath and -s -w.

Model: opus-5-5

script/assets

@@ -2,8 +2,8 @@
 # script/assets: extract Alpine.js from its npm package tarball, committed
 # in 3p/, to static/js/alpine.min.js, where go:embed reads it. The package
 # is @alpinejs/csp, Alpine's build for pages whose Content-Security-Policy
-# forbids eval. The extracted file is not committed. script/test, make
-# build and make dev run this first.
+# forbids eval. The extracted file is not committed. make build, make dev
+# and the Dockerfile's lint, test and build stages run this first.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"