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script/ci-mark-superseded, so it and its tests are removed.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

@@ -1,7 +1,8 @@
 #!/bin/sh
 # script/fmt: format all files (writes): the Go code with gofmt and
 # goimports, the Markdown with prettier. prettier is never installed
-# locally: it runs in docker, in the Dockerfile's Markdown stages.
+# locally: it runs in docker, in the Dockerfile's Markdown stages, built
+# with --no-cache like every docker build in script/.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -12,7 +13,8 @@ main() {
     if command -v goimports >/dev/null 2>&1; then
         goimports -w .
     fi
-    docker build --target markdown-output --output type=local,dest=. .
+    docker build --no-cache \
+        --target markdown-output --output type=local,dest=. .
 }
 
 main "$@"