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. golangci-lint run also
mounts golangci-lint's cache. The test phase and the build stage
install their packages with apt-get and apk directly, as
REPO_POLICIES.md now has them do, instead of through
script/bootstrap --cgo. The test phase keeps Debian's recommended
packages, which bring libheif's AVIF encoder that the tests use.

Model: opus-5-5

README.md

@@ -825,19 +825,24 @@ provide:
   `script/precommit`
 
 Every `docker build` in these scripts passes a new value of the `CACHEBUST`
-build argument, which every stage of the `Dockerfile` declares first, so every
-`RUN` step, the lint and test phases included, runs on every build instead of
-coming from the build cache. Go's build cache is kept from build to build: the
-steps that compile mount it, so a package whose inputs are unchanged is not
-compiled again, and both `go test` runs pass `-count=1` so that no stored test
-result is replayed. The `test` phase runs on the Debian Go image, with Debian's
-libvips and libheif; the image pixa ships is built and runs on Alpine.
-`script/check`, `script/cibuild`, `script/docker`, `script/lint`, `script/test`,
-`script/setup` and `script/install-precommit` are the standard copies from
-`sneak/prompts`, kept identical to them. `script/fmt` and `script/fmt-check` are
-the standard copies with pixa's `gofmt` step kept before prettier. prettier
-formats the markdown only, not the HTML templates, as it cannot parse a Go
-template action inside a tag.
+build argument, which every stage of the `Dockerfile` declares, so every `RUN`
+step after that line, the lint and test steps included, runs on every build
+instead of coming from the build cache. Above the line are the steps that
+install system packages and download the Go modules; Docker keeps their layers
+until the base image, the step itself, `go.mod` or `go.sum` changes. Go's build
+cache is kept from build to build: the steps that compile mount it, so a package
+whose inputs are unchanged is not compiled again, and both `go test` runs pass
+`-count=1` so that no stored test result is replayed. The lint step also mounts
+golangci-lint's cache, which reuses a package's findings only when its files,
+those of the packages it imports, `go.mod`, `.golangci.yml` and the
+golangci-lint version are all unchanged. The `test` phase runs on the Debian Go
+image, with Debian's libvips and libheif; the image pixa ships is built and runs
+on Alpine. `script/check`, `script/cibuild`, `script/docker`, `script/lint`,
+`script/test`, `script/setup` and `script/install-precommit` are the standard
+copies from `sneak/prompts`, kept identical to them. `script/fmt` and
+`script/fmt-check` are the standard copies with pixa's `gofmt` step kept before
+prettier. prettier formats the markdown only, not the HTML templates, as it
+cannot parse a Go template action inside a tag.
 
 ## Load Test