cibuild: run checks on unchanged trees via CHECK_EPOCH (closes #89)

A bare `docker build .` keyed `make fmt-check-go`, `make lint`, the
prettier check, and `make test` on the build context, so on an unchanged
tree every check layer was a cache hit: the build exited 0 in under a
second having run none of them.

Add `ARG CHECK_EPOCH` immediately above the first check RUN in all three
check stages (lint, mdfmt, builder), and have script/cibuild pass a fresh
`--build-arg CHECK_EPOCH` each run. The changing value busts the cache from
that point down, while `go mod download` and `yarn install` above it stay
cached, so the build does not regress to cold. The false claim in
script/cibuild's header comment is corrected.

Running the checks for real surfaces the pre-existing intermittent
internal/cli test timeout (the gpg-subprocess flake in #62 / #67); that
defect is out of scope here.

Model: opus-4-8

TODO.md

@@ -24,6 +24,8 @@ only thing left of the `chore/align-repo-policies` branch is the list below.
 
 # Completed Steps
 
+- 2026-09-21: made `script/cibuild` re-run the checks on an unchanged tree via a
+  changing `CHECK_EPOCH` build arg in each Dockerfile check stage (#89)
 - 2026-10-02: a plain `docker build .` of a clone now stamps the tag or short
   commit into `mfer version` instead of nothing: `.dockerignore` sends `.git`
   (not `.git/config`), and the build stage takes the `VERSION` build argument,