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. script/lint, script/test and
REPO_POLICIES.md come from its next at c55a0cb, so the lint and test
builds write no image. golangci-lint is v2.14.0 and raises no findings.
Lint and test are phases of the Dockerfile; 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.
Formatting runs on the host: script/bootstrap installs the pinned node and
yarn, and the prettier and markdown stages are gone. .claude/settings.json
is deleted.

Deviation: the workflow keeps fetch-depth: 0.
Deviation: .gitignore keeps the scan database patterns.
Over the cap: make test takes 82 to 100 seconds on this host.

Model: opus-5-5

Makefile

@@ -6,7 +6,7 @@ BINARY := sfdupes
 VERSION := $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
 LDFLAGS := -X main.Version=$(VERSION)
 
-.PHONY: sfdupes build bootstrap setup test test-race lint fmt fmt-check check docker hooks clean
+.PHONY: sfdupes build bootstrap setup test lint fmt fmt-check check docker hooks clean
 
 # Standard targets are thin shims; the implementations live in script/
 # per the scripts-to-rule-them-all pattern.
@@ -27,9 +27,6 @@ setup:
 test:
 	@script/test
 
-test-race:
-	@script/test-race
-
 lint:
 	@script/lint