Re-vendor the canonical files from sneak/prompts at 0653579 (closes #221)

REPO_POLICIES.md, script/cibuild, script/docker, script/lint and
script/test are their copies at that commit. In every Dockerfile stage
the system package install and the Go module download now come before
ARG CACHEBUST, so check builds keep those layers until the base image,
the install line, go.mod or go.sum change; every step that lints, tests
or compiles still reruns. golangci-lint run also mounts golangci-lint's
cache. The stages install their packages with apt-get and apk directly
instead of through script/bootstrap --cgo. The test phase installs no
recommended packages and names libheif's AVIF encoder,
libheif-plugin-aomenc, which the tests use.

Model: opus-5-5

script/bootstrap

@@ -19,9 +19,8 @@
 #                            CGO image libraries (pkg-config, vips
 #                            with its JPEG XL support, libheif) for
 #                            the govips bindings instead of Node: to
-#                            compile pixa, in the Dockerfile's test
-#                            phase and build stage, which format
-#                            nothing
+#                            compile pixa outside Docker, as
+#                            `make build` does
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"