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script/ci-mark-superseded, so it and its tests are removed.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

@@ -60,10 +60,10 @@ main() {
     if missing go; then pkg_install go golang go go; fi
 
     # Not installed here: docker is platform-specific and out of scope for a
-    # package-manager bootstrap, but script/lint, script/fmt and script/css
-    # need it.
+    # package-manager bootstrap, but script/test, script/lint, script/fmt and
+    # script/css need it.
     if missing docker; then
-        echo "bootstrap: docker not found; script/lint, script/fmt and script/css require it" >&2
+        echo "bootstrap: docker not found; script/test, script/lint, script/fmt and script/css require it" >&2
     fi
 
     go mod download