feat: stage neutral repo contract

[lnittman]

Summary

• add the neutral agent-plugin-companion repository identity
• declare the intentionally slim plugin-distribution root shape and product-runtime omissions
• keep plugin generation and runtime ownership unchanged

Verification

• neutral-only validation passed with @creative-int/workspace@0.13.0
• git diff --check
• repo-owned generated/type/test/smoke verification was run

This companion has no legacy template.contract.json; its genesis evidence is recorded by the repo-contract convergence campaign rather than inventing a fake migration source.