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, run as the
node user, which the not-writable-directory tests need), 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

TODO.md

@@ -18,6 +18,13 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-23: Re-vendored the lint and test setup from the template (issue 96).
+  Linting and testing are the `lint` and `test` phases of the `Dockerfile`;
+  `script/lint` and `script/test` each build one with `--no-cache`, and the last
+  stage compiles and depends on both, so `script/cibuild` is one build.
+  `Dockerfile.lint`, `CHECK_EPOCH`, `LINT_EPOCH` and the tests that checked them
+  are gone; `REPO_POLICIES.md` is re-copied.
+
 - 2026-09-23: Stopped `helper fix-missing-thumbnails` retrying files the server
   always refuses (issue 109). Both thumbnail helpers skip a file another account
   owns without fetching it. The fixer skips a file whose recorded thumbnail size