fix: checkout authorship helpers from github repo

[haasonsaas]

Summary

• stop using the caller repository workflow SHA when the reusable authorship workflow checks out evalops/.github helper scripts
• add a helper_ref input that defaults to main so existing callers keep working, while callers can pin helper script refs explicitly

Verification

• actionlint .github/workflows/agent-authorship-label.yml
• ruby -c .github/scripts/classify-agent-authorship.rb

[cursor]

PR Summary

Low Risk
Low risk: small change limited to a reusable GitHub Actions workflow, with a default preserving current behavior; failures would mainly impact labeling automation if a bad helper_ref is supplied.

Overview
Stops checking out evalops/.github helper scripts at github.workflow_sha and instead adds a new workflow_call input, helper_ref (default main), to control which ref is used.

This lets callers explicitly pin the helper scripts version while keeping existing callers working unchanged.

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