refactor: extract runtime policy contracts

[chubes4]

Summary

• Extract runtime policy contracts, validation errors, and policy assertion helpers from packages/runtime-core/src/index.ts into runtime-policy.ts.
• Re-export the new module from the runtime-core public barrel so existing imports keep working.

Tests

• npm run build
• npm run policy-validation-smoke
• npm run command-registry-smoke
• npm run package-distribution-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the focused runtime-core extraction, ran verification, and prepared the PR body for Chris to review.