Lint and test as phases of the Dockerfile (closes #38)

Lint and test are now phases of the one Dockerfile, as the current repo policy requires: a lint phase on the pinned golangci-lint image and a test phase on the pinned Go image, and the build stage depends on both, so a plain docker build . fails when either fails. Dockerfile.lint is gone. REPO_POLICIES.md and script/lint, test, docker and cibuild are byte-identical to the current sneak/prompts copies, so every docker build in script/ is uncached and tagged. make test now needs Docker on the host; formatting is checked on the host only.

Judgement calls: the test phase installs gcc and musl-dev unpinned for -race; no -count=1, since a build stage holds no earlier result.

Model: opus-5-5

script/bootstrap

@@ -3,9 +3,9 @@
 # repo. Idempotent: every install is guarded by a check, so tools that
 # are already there are left alone. Base tooling comes from nix, apt,
 # brew, or apk, detected in that order, and nothing is assumed to be
-# present. The linter is not installed here: it only ever runs inside
-# the image built from Dockerfile.lint, so Docker is what is needed for
-# it, and that is checked for rather than installed.
+# present. The linter is not installed here: linting and testing run
+# only as phases of the Dockerfile, so Docker is what is needed for
+# them, and that is checked for rather than installed.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -60,7 +60,7 @@ main() {
     go mod download
 
     if missing docker; then
-        echo "bootstrap: docker is not installed; make lint needs it" >&2
+        echo "bootstrap: docker is not installed; make lint and make test need it" >&2
     fi
 
     echo "bootstrap complete"