docs: establish platform reference profile registry

[chuks]

Summary

Establishes the platform-neutral home and governance contract for executable Ratify reference profiles.

This PR deliberately contains no Google, NVIDIA, LangChain, XMTP, or other platform implementation or registry entry. After it merges, every platform reference can branch independently from main, add exactly one profile directory and registry row, and merge without depending on another platform PR.

Changes

• Adds references/README.md with the profile contract, empty registry, lifecycle, and endorsement-status rules.
• Links reference profiles from the root README and repository tree.
• Clarifies the boundary between open interoperability references and managed Ratify Verify adapters.

Verification

• git diff --check
• Reviewed against open drafts feat: add NOOA and OpenShell delegated-authority reference #61, feat: add Google ADK delegated-authority reference profile #63, and feat: add LangChain delegated-authority reference profile #66 to ensure all platform-specific content remains in those PRs.

Follow-up

After merge, the existing platform branches will be rebased onto main, their PR bases normalized to main, and their platform-specific gates rerun.