Address P1 lint finding: structured eval-dispatch comments could be
forged by unauthorized users. Add three-layer consumer-side validation:
1. comment.user.login == 'github-actions[bot]' (platform-enforced identity)
2. comment.performed_via_github_app.slug == 'github-actions' (App signature)
3. payload.run_id verified against actual successful workflow run via API
Also adds:
- eval_poll_validate.py: consumer validation module (in-repo, auditable)
- test_eval_poll_validate.py: unit tests proving forged comments are rejected
- Go security contract test updated to assert run_id and validate reference
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
