Build GitHub Action runner for PR evidence

[fraware]

Goal

Replace the placeholder action.yml behavior with a real OVK PR runner.

Scope

• Add ovk ci --mode advisory|enforce|attest.
• Generate ovk-evidence.json.
• Render markdown suitable for a PR comment.
• Map merge recommendations to exit codes.
• Upload evidence as an artifact in GitHub Actions.

Acceptance criteria

• Enforce mode exits non-zero on block.
• Advisory mode comments or prints evidence without failing.
• Unknown-like outcomes require human review in enforce mode.
• Evidence is bound to the evaluated commit SHA when available.