ci: relay monorepo releases for legacy clients

[ZhiXiao-Lin]

Summary

• add a manual compatibility relay from canonical A3S-Lab/a3s releases
• verify the exact asset set, GitHub digests, checksum manifests, and inert four-file legacy bridge before publishing
• mirror assets byte for byte into an owned GitHub-only release without crates.io or Homebrew credentials
• mark this repository as the retained compatibility endpoint and direct active development to the monorepo

The workflow uses this repository's GITHUB_TOKEN to create the compatibility tag, so that tag creation cannot recursively trigger the retired tag release workflow. The old workflow would also reject the relayed tag because it does not match this repository's frozen manifest version.

Validation

• go run github.com/rhysd/actionlint/cmd/actionlint@v1.7.7 .github/workflows/relay-monorepo-release.yml
• extracted both workflow shell programs and validated them with bash -n
• Ruby/Psych YAML parse
• git diff --check