Add public API governance instruction evaluation

[PureWeen]

Summary

• adds a specialized Vally regression suite for the repository-wide public API governance instructions in Add public API governance instructions #68168
• covers an unapproved new public member, shipped interface signature change, and shipped default/convention change
• loads .github/copilot-instructions.md into Vally's isolated execution environment and checks the complete API-review gate

This is the top layer over #68168 and targets pureween-public-api-instructions. It follows the evaluation infrastructure merged in #68628 and adds no Vally onboarding infrastructure.

Validation

Validated in a synthesized checkout containing current main's #68628 infrastructure, #68168's instruction changes, and this stack layer:

• pwsh -NoProfile -File ./eng/skill-evals/run.ps1 Test
• pwsh -NoProfile -File ./eng/skill-evals/run.ps1 Validate -Eval eng/skill-evals/copilot-instructions/public-api-governance.vally.yaml
• one current-behavior run with --runs 1 --workers 3: 100%, all graders passed for all three stimuli
• git diff --check