ci(release): split metadata prep from publish

[cbusillo]

Summary

• skip full preflight and release matrix while preparing release metadata
• build only the Linux x86_64 Code binary needed for changelog/release-note generation during metadata prep
• keep full preflight, macOS/Linux builds, and Windows release assets on the publish pass
• document the two-phase release behavior

Validation

• actionlint .github/workflows/release.yml
• ruby -e 'require "yaml"; YAML.load_file(ARGV[0]); puts "yaml ok"' .github/workflows/release.yml
• ./build-fast.sh