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script/ci-mark-superseded, so it and its tests are removed.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

@@ -1,14 +1,15 @@
 #!/bin/sh
 # script/css-check: fail when static/css/tailwind.css differs from what
 # script/css would generate (read-only). The comparison is the Dockerfile's
-# css-check stage, which the image build runs too.
+# css-check stage, which the image build runs too. --no-cache because a
+# cached check is a check that did not run.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build --target css-check --output type=cacheonly .
+    docker build --no-cache --target css-check --output type=cacheonly .
 }
 
 main "$@"