GitHub Copilot: re-verify COP-* coverage for AGENTS/custom-instructions support

[avifenesh]

Summary

GitHub Copilot docs/changelog include AGENTS/custom-instructions support and path-specific instruction behavior. We should re-verify COP-* coverage and evidence freshness.

Sources:

• https://docs.github.com/en/copilot/reference/custom-instructions-support
• https://docs.github.com/en/copilot/how-tos/custom-instructions/adding-repository-custom-instructions-for-github-copilot
• https://github.blog/changelog/2025-06-17-github-copilot-coding-agent-now-supports-agents-md-custom-instructions/

Proposed Work

• Re-verify COP-* assumptions (global/scoped instructions, applyTo semantics, custom agents/hooks/setup workflow).
• Refresh evidence metadata (verified_on, URLs).
• Add tests only for actual behavior changes.

Acceptance Criteria

• COP rule evidence is current and traceable to official docs.
• No stale support assertions remain.
• Parity/tests pass.