[codex] Add downstream consumer smoke test

[timmyyuan]

Summary

• add a downstream consumer smoke script that creates a temporary external Go module
• validate the generated IR with FileCheck
• run the smoke test in the Dockerized cgo CI job
• document the workflow in README and CHANGELOG

Validation

• bash -n scripts/test-downstream-consumer.sh
• CGO_ENABLED=0 GOCACHE=/private/tmp/mlir-go-gocache-noncgo go test -count=1 ./...
• docker build --build-arg LLVM_VERSION=22 --tag mlir-go-dev . (blocked locally by Docker Hub token request timeout; GitHub Actions runs the Dockerized cgo path)

Fixes #12