Make lint and test phases of the Dockerfile (closes #96)

Follows the template: Dockerfile.lint is gone; the Dockerfile has a lint
phase (eslint, prettier --check .) and a test phase (vitest), and its last
stage compiles and depends on both. script/lint and script/test build one
phase each with --no-cache; script/docker and script/cibuild pass
--no-cache, so CHECK_EPOCH and LINT_EPOCH are removed. script/cibuild is
the single image build, so CI runs lint and the tests once each. The tests
that checked the old layout are deleted, REPO_POLICIES.md is re-copied and
the README describes the new layout.

Model: opus-5-5

.dockerignore

@@ -1,6 +1,6 @@
 # Mirrors .gitignore, with one deliberate exception: .gitignore itself stays
 # in the build context, because prettier 3 reads it as a default ignore file
-# and dropping it would change what `make fmt-check` sees inside the image.
+# and dropping it would change what the lint phase's prettier check sees.
 
 # VCS
 .git