cibuild: the org model, which runs every check uncached (closes #37)

script/cibuild was a plain docker build ., so on a tree Docker had
seen before every check step came from the build cache and the build
still passed. It is now the org model from sneak/prompts, byte for
byte: it runs script/bootstrap and script/check, then builds the image
with --no-cache and the git describe version as the VERSION build
argument, which the Dockerfile's builder stage already declares.
README.md's entry for the script says what it now does.

Model: opus-5-5

TODO.md

@@ -23,6 +23,12 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: CI can no longer pass on checks that did not run (issue #37):
+  `script/cibuild` is now the org model, byte for byte. It runs
+  `script/bootstrap` and `script/check`, then builds the image with `--no-cache`
+  and the version from `git describe` as the `VERSION` build argument, where it
+  used to be a plain `docker build .` whose check steps could come from the
+  build cache
 - 2026-09-29: nginx sends the security headers `REPO_POLICIES.md` requires on
   every response (issue #18), including errors, `/assets/` and what it passes on
   from the backend, whose own copies it drops so each header goes out once. They