Run lint and tests as Dockerfile phases built with --no-cache (closes #202)

script/check, cibuild, docker, lint, test, setup and install-precommit
are now the copies from sneak/prompts main, unchanged: lint and test
each build their Dockerfile phase with --no-cache, and cibuild runs
bootstrap and check before building the image. The lint phase runs
golangci-lint from the image REPO_POLICIES.md names, with libvips-dev
from apt-get; the test phase runs the tests with a 90-second timeout;
the build stage depends on both. Dockerfile.lint and CHECK_EPOCH are
gone, the formatting check stays on the host, and make docker-versioned
and docker-test call the scripts. Without VERSION the stage that
compiles still takes the version from git describe, per issue 166.

Model: opus-5-5

README.md

@@ -568,22 +568,31 @@ them. We provide:
 - `script/setup` — make a fresh clone ready for development
   (bootstrap, then install-precommit)
 - `script/projectname` — output the project name ("pixa")
-- `script/test` — run the test suite
-- `script/lint` — run golangci-lint, always in a container (builds
-  `Dockerfile.lint` when run outside one)
+- `script/test` — run the test suite: build the `test` phase of the
+  `Dockerfile`, tagged `pixa-test`
+- `script/lint` — run golangci-lint: build the `lint` phase of the `Dockerfile`,
+  tagged `pixa-lint`; the linter never runs on the host
 - `script/fmt` — format all code (writes)
-- `script/fmt-check` — check formatting (read-only)
+- `script/fmt-check` — check formatting (read-only), on the host
 - `script/check` — run test, lint, and fmt-check
-- `script/docker` — build the Docker image tagged via `script/projectname`
+- `script/docker` — build the Docker image tagged via `script/projectname`, with
+  the version from `git describe`; the image's build stage depends on the `lint`
+  and `test` phases, so this runs them too
 - `script/docker-smoke` — build the image, start it, wait for it to be healthy
-- `script/cibuild` — CI entrypoint: `docker build .` with a new
-  `CHECK_EPOCH` on every run, so the Dockerfile's checks run instead of
-  coming from the build cache, and a green run implies a green repo
+- `script/cibuild` — CI entrypoint: run `script/bootstrap` (which installs Go
+  and the libvips libraries on the host), then `script/check`, then build the
+  image as `script/docker` does
 - `script/precommit` — pre-commit checks (`go mod tidy` guard, then
   `script/check`)
 - `script/install-precommit` — install the git pre-commit hook that
   runs `script/precommit`
 
+Every `docker build` in these scripts passes `--no-cache`, so the lint and test
+phases run on every build instead of coming from the build cache.
+`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.
+
 ## TODO
 
 See [TODO.md](TODO.md) for the full prioritized task list.