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/test

@@ -1,17 +1,19 @@
 #!/bin/sh
-# script/test: run the test suite. Reruns verbosely on failure so CI
-# logs show which test failed.
+# script/test: run the test suite. Testing is a phase of the Dockerfile
+# and this builds that phase alone, on the same terms as script/lint:
+# --target because a phase that is not the last stage is built only when
+# named, --no-cache because a cached test layer is a test that did not
+# run, and a tag so each build replaces the previous image.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    go test -timeout 30s -cover ./... || {
-        echo "--- Rerunning with -v for details ---"
-        go test -timeout 30s -v ./...
-        exit 1
-    }
+    docker build --no-cache \
+        --target test \
+        -t "$("$SCRIPT_DIR/projectname")-test" .
 }
 
 main "$@"