Add Codebox package boundary guardrails

[chubes4]

Summary

• Add a static production boundary check that keeps runtime-core from depending on or importing runtime backend packages, WP Playground SDK modules, or Playwright.
• Keep public API docs focused on Codebox-owned entrypoints and generic internal adapter language.
• Update docs boundary assertions to enforce the generic public contract wording.

Tests

• npm run test:production-boundary-enforcement
• npm run test:public-api-contract
• npm run test:runtime-contract-package-exports
• npm run test:docs-boundary-language

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Scoped boundary test implementation and verification