Build the EternalTerminal `et` client in CI the way mosh-client is:
- build-et-binaries.yml builds et for linux x64/arm64, macOS universal
and windows x64, uploads artifacts, and optionally publishes them to a
dedicated binary repository on manual workflow_dispatch.
- scripts/build-et/{linux,macos,windows} compile et from upstream.
- .gitignore excludes the fetched binaries; resources/et/README.md
documents source provenance.