Re-vendor the canonical files from sneak/prompts at dd4027b (closes #121)

The vendored files are copies from sneak/prompts dd4027b, with this
repository's own entries after the canonical content. golangci-lint is
v2.14.0. Lint and test are phases of the Dockerfile, which script/lint
and script/test build with --no-cache; Dockerfile.lint and
script/lint-darwin are gone, and the lint phase also checks the macOS
build. The tests run on the Debian Go image with cgo and the race
detector. script/cibuild bootstraps, runs script/check and builds the
image; CHECK_EPOCH and the memlock ulimit are gone. Go's build cache
stays in a cache mount, out of the test image's layer. Agent guidance
lives in AGENTS.md alone; the tool-specific file is gone.

Model: opus-5-5

Makefile

@@ -1,7 +1,7 @@
 export CGO_ENABLED=1
 
-.PHONY: default bootstrap setup build test lint lint-darwin fmt fmt-check \
-	check docker docker-run clean install hooks
+.PHONY: default bootstrap setup build test lint fmt fmt-check check docker \
+	docker-run clean install hooks
 
 default: check
 
@@ -21,13 +21,10 @@ test:
 fmt:
 	@script/fmt
 
+# Vets and lints the Linux build, then the macOS build
 lint:
 	@script/lint
 
-# Type-check and lint the macOS build from Linux (see script/lint-darwin)
-lint-darwin:
-	@script/lint-darwin
-
 check:
 	@script/check