feat: add @depot/sandbox package

[ischolten]

Summary

Add a beta @depot/sandbox package under packages/sandbox. The package provides a TypeScript client surface for creating sandboxes, running commands, streaming command output, and using a sandbox-bound filesystem API.

This also converts the repo to a pnpm workspace so the existing root @depot/sdk-node package and the new sandbox package can be checked independently, while keeping the root package intact.

Testing

• pnpm fmt:check
• pnpm type-check:all
• pnpm test:all
• pnpm build:all
• npm pack --dry-run from packages/sandbox

[linear-code]

DEP-4140