ci: run authorship labels off github hosted budget

[haasonsaas]

Summary

• Move the reusable agent-authorship label job from ubuntu-latest to the org Blacksmith runner by default, with a reusable-workflow input for overrides.
• Checkout helper scripts at github.workflow_sha so pinned reusable workflow calls execute matching helper code instead of mutable main.

Verification

• ruby -c .github/scripts/classify-agent-authorship.rb
• git -c core.fsmonitor=false diff --check
• actionlint .github/workflows/agent-authorship-label.yml

[cursor]

PR Summary

Medium Risk
Medium risk because it changes CI execution environment (runs-on) and the ref used to fetch org helper scripts, which can affect workflow reliability if the runner label or SHA resolution is misconfigured.

Overview
Moves the reusable agent-authorship-label workflow off ubuntu-latest by introducing a runner_label input (defaulting to blacksmith-4vcpu-ubuntu-2404) and using it for runs-on.

Pins the checkout of evalops/.github helper scripts to ${{ github.workflow_sha }} instead of main, ensuring helper code matches the exact reusable workflow revision being executed.

^{Reviewed by Cursor Bugbot for commit a1ffa0d. Bugbot is set up for automated code reviews on this repo. Configure here.}