Re-vendor the canonical files from sneak/prompts at dd4027b (closes #95)

The shared files are the copies at sneak/prompts commit dd4027b, with this
repository's own entries kept after them. golangci-lint is v2.14.0 and
raises no findings. Lint and test are phases of the Dockerfile, built by
script/lint and script/test; the tests run under the race detector as
nobody, so Dockerfile.lint, script/verify-lint-image-pin and make
test-race are gone. Every docker build in script/ passes --no-cache in
place of the old cache-busting build argument, and script/cibuild runs
script/bootstrap and script/check before the image build.
.claude/settings.json is deleted.

Deviation: the workflow keeps fetch-depth: 0 for the tag-derived version.
Deviation: .gitignore keeps the scan database patterns.
Deviation: prettier still runs in Docker, not on the host.
Over the cap: make test takes about 60 seconds on this host.

Model: opus-5-5

script/fmt

@@ -2,7 +2,8 @@
 # script/fmt: format all files (writes): the Go sources with gofmt, the
 # Markdown with prettier. prettier is never installed on the host: it
 # runs from the Dockerfile's prettier stage with the repository mounted,
-# as the calling user so the files it rewrites keep their owner. The tag
+# as the calling user so the files it rewrites keep their owner. The
+# build passes --no-cache, as every docker build in script/ does. The tag
 # makes each build replace the previous image instead of leaving another
 # one behind.
 set -eu
@@ -14,7 +15,7 @@ main() {
     cd "$ROOT"
     gofmt -s -w .
     image="$("$SCRIPT_DIR/projectname")-prettier"
-    docker build -q --target prettier -t "$image" . >/dev/null
+    docker build -q --no-cache --target prettier -t "$image" . >/dev/null
     docker run --rm --user "$(id -u):$(id -g)" -v "$ROOT:/src" "$image" \
         prettier --write '**/*.md' --tab-width 4 --prose-wrap always
 }