Go (builds the zot agent) and Rust (builds the Colibri release binaries) are
required on the build host to produce the binaries build.sh stages, but were
undocumented. Add them to REQUIREMENTS.md (build-host only, not the image), and
make the binary-missing preflights note when the matching toolchain (go/cargo)
isn't installed so that case surfaces up front instead of later.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
