Local-only builds by default for build-devenv and
build-imagemagick-docker-image; --push is now required to build
multi-platform and push to the registry. All release-image build
commands (frontend, backend, exporter, mcp, storybook) now accept
--tag to override the image tag. Adds a shared DEVENV_TAG variable
threaded through pull-devenv, the production build function and
docker-compose.main.yml so a custom devenv tag can be used
end-to-end.