Add GitHub Copilot delegated authority reference

[chuks]

Summary

Adds an independent GitHub Copilot and MCP reference for receiver-verifiable Ratify authority.

• native Copilot plugin, skill, and bundled MCP adapter
• independent protected receiver
• exact repository, resource, payload, invocation, freshness, revocation, replay, and trust-root enforcement
• developer-first README with Mermaid architecture visuals
• self-contained clean-install runtime
• explicit independent-reference and production-limit disclosures

Verification

• seven reference tests pass with zero failures or skips
• clean npm install and deterministic bundle checksum
• clean plugin directory with no node_modules or TypeScript executed through Copilot CLI 1.0.80
• repository-wide make test-all passes across Go, TypeScript, Python, Rust, C/C++, wire equivalence, fixtures, and release sync
• git diff --check

This reference does not change protocol or SDK versions and does not initiate an alpha.17 release.