docs: deprecate standalone service-runtime upstream

[haasonsaas]

Summary

• mark evalops/service-runtime as a deprecated standalone upstream now that Platform owns pkg/, cmd/, and service-local runtime integration
• 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 and GitHub metadata changes only; no runtime code or release behavior is modified.

Overview
Marks evalops/service-runtime as a deprecated standalone upstream and directs contributors to start new runtime work in evalops/platform (linking evalops/platform#1768).

Adds repo-level routing/guardrails: a GitHub issue template config that disables blank issues and points to Platform, a PR checklist to prevent new source-of-truth work here, and a new RETIREMENT.md describing allowed vs. disallowed changes and the contributor flow during retirement.

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