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/cibuild

@@ -2,11 +2,12 @@
 # script/cibuild: run the CI build. It bootstraps first: a CI runner
 # checks out and runs this and nothing else, and script/fmt-check runs
 # the formatter on the host, which a pristine checkout cannot do.
-# A new CACHEBUST value reruns every RUN step, since a cached gate phase
-# is a check that did not run; unlike --no-cache, it keeps Go's build cache.
+# A new CACHEBUST value reruns every RUN step after the ARG CACHEBUST
+# line, since a cached gate phase is a check that did not run; unlike
+# --no-cache, it keeps the layers above that line and the cache mounts.
 # This relies on every stage of the Dockerfile declaring ARG CACHEBUST
-# first; a stage without that line is served from cache on an unchanged
-# tree, with no warning.
+# before every step that lints, tests or compiles; a stage without that
+# line is served from cache on an unchanged tree, with no warning.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"