Initialize code mode sandbox package

[ggoodman]

Summary

• initialize the @torkbot/code-mode-sandbox package boilerplate
• add TypeScript build, typecheck, test, and package configuration
• add cross-platform CI, repository guidance, README, MIT license, and an empty entrypoint
• build a SHA-bound npm tarball for every successful main commit and publish releases from that exact artifact without rebuilding

Scope

This intentionally contains no Sandbox adapter implementation, runtime API, or speculative dependencies. Those will be designed and implemented separately.

Validation

• npm run typecheck
• npm test
• npm run build
• npm pack --dry-run
• local release manifest write/verify round trip