Ship a versioned GitHub Action prerelease

[BLCCoreStudio]

Goal

Move the new composite GitHub Action from development-line evaluation (@main) to a versioned prerelease that users can reference explicitly.

Acceptance criteria

• cut a prerelease containing the current action.yml and verified action-entrypoint.sh
• provide a versioned Action ref and update README examples away from @main
• keep the runtime binary version and fixed SHA-256 digest explicit
• keep Linux x86_64 and early-alpha limitations visible
• run the Action smoke test against the release candidate behavior before publication
• document report-only mode and optional fail-on-conflict gating in release notes
• retain read-only repository permissions in the documented workflow
• recommend immutable commit-SHA pinning for higher-trust production use

Follow-up

Evaluate GitHub Marketplace publication only after the versioned Action path is stable and its metadata/first-run experience are validated. Marketplace presence should not be treated as a substitute for correctness or adoption.

This tracks the README roadmap item for a versioned GitHub Action release.