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script/ci-mark-superseded, so it and its tests are removed.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

@@ -3,8 +3,7 @@
 # Docker: Dockerfile.browser builds the test and runs it in a digest-pinned
 # headless browser image, so the host needs no browser.
 #
-# --no-cache-filter=browser runs the test again even when nothing changed;
-# it must name the stage in Dockerfile.browser that runs it.
+# --no-cache because a cached test layer is a test that did not run.
 # --output=type=cacheonly leaves no image behind to clean up.
 set -eu
 
@@ -14,7 +13,7 @@ main() {
     cd "$ROOT"
     docker build \
         -f Dockerfile.browser \
-        --no-cache-filter=browser \
+        --no-cache \
         --progress=plain \
         --output=type=cacheonly \
         .