feat: add post-codefreeze-gatekeeper evaluation workflow

[MohammadiIram]

Summary

• Adds the thin caller workflow (.github/workflows/post-codefreeze-gatekeeper.yaml) that delegates to the central reusable workflow in devops-shared-resources
• The gatekeeper runs in evaluation (advisory-only) mode -- it posts a PR comment with pass/fail results but does not block merges
• Teams can use the run-gatekeeper label to manually re-trigger the check

Resolves: RHOAIENG-82298

[github-actions]

Post-Codefreeze Gatekeeper — PASSED (evaluation)

Advisory only — this check does not block merging. Please review the results before merging.

Target branch: rhoai-3.5
Required Jira version: 3.5 GA RHOAI RELEASE

✅ All checks passed.

• RHOAIENG-82298: All checks passed

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_{After resolving issues, push a commit, re-edit the PR description, or add the run-gatekeeper label to re-trigger this check.}

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jstetina, MohammadiIram
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