check / check (push) Failing after 1s
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