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script/ci-mark-superseded, so it and its tests are removed.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/check

@@ -1,7 +1,7 @@
 #!/bin/sh
 # script/check: run all checks (test, lint, fmt-check, css-check). Our own
-# extension to scripts-to-rule-them-all.
-# Writes only the ignored static/js/alpine.min.js, through script/test.
+# extension to scripts-to-rule-them-all. test, lint and css-check are
+# Docker builds; fmt-check runs gofmt on the host. Writes nothing.
 # Generic, apart from css-check.
 set -eu