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 the golangci-lint
v2.14.0 image, which also runs the js-lint stage, and Dockerfile.lint is
gone. Tests move into a test phase on the golang bookworm 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 The workflow no longer calls script/ci-mark-superseded, so it and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/fmt-check

@@ -1,6 +1,7 @@
 #!/bin/sh
 # script/fmt-check: check formatting (read-only). Same scope as
-# script/fmt, but fails instead of writing.
+# script/fmt, but fails instead of writing. --no-cache because a cached
+# check is a check that did not run.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -12,7 +13,7 @@ main() {
         gofmt -s -l .
         exit 1
     fi
-    docker build --target markdown-check --output type=cacheonly .
+    docker build --no-cache --target markdown-check --output type=cacheonly .
 }
 
 main "$@"