docs: deprecate standalone proto upstream

[haasonsaas]

Summary

• mark evalops/proto as a deprecated standalone upstream now that Platform owns proto/ and gen/
• add RETIREMENT.md with allowed changes, non-goals, and contributor flow
• route new issues and PRs back to evalops/platform#1768

Refs evalops/platform#1768

Verification

• git diff --check

[cursor]

PR Summary

Low Risk
Low risk: documentation/repo-metadata updates only, routing contributors to evalops/platform with no code, schema, or release logic changes.

Overview
Deprecates evalops/proto as the active protobuf source of truth and points contributors to evalops/platform (proto/ and gen/) for all new contract work.

Adds repository guardrails: disables blank GitHub issues in favor of Platform contact links, introduces a PR checklist to prevent new source-of-truth changes here, and adds RETIREMENT.md documenting allowed/forbidden changes and the retirement flow (tracking evalops/platform#1768).

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