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/bootstrap

@@ -68,9 +68,9 @@ main() {
     # Warn, do not fail: only the targets named below, and the
     # pre-commit hook, need docker.
     if missing docker; then
-        echo "bootstrap: WARNING: docker not found; make lint, make fmt," >&2
-        echo "bootstrap: make fmt-check, make check, make docker and" >&2
-        echo "bootstrap: make test-race require it." >&2
+        echo "bootstrap: WARNING: docker not found; make test, make lint," >&2
+        echo "bootstrap: make fmt, make fmt-check, make check and" >&2
+        echo "bootstrap: make docker require it." >&2
     fi
 
     go mod download