[sandbox-cli] Add stdin, output, domain, and user controls

[erulkey]

The SDK can pipe input to a command, keep detached stdin open, read one output stream, resolve a published port, and manage Linux users and groups. The CLI could not call those operations directly. This adds matching commands and examples to the repository Sandbox skill.

For a foreground command, printf 'hello\n' | sandbox exec --stdin my-sandbox -- cat forwards input, streams output, and returns the command's exit code in one invocation. For a command that needs input later, sandbox exec --detach --stdin prints its ID; sandbox commands write sends bytes from another CLI process, close-stdin sends EOF, and output --stream stdout|stderr|both reads its output. sandbox domain returns a published port's URL. sandbox users and sandbox groups create users and shared groups, run as a user, and change membership.

The command controls use the current running session so an old command ID cannot silently target a resumed session. User and group commands call the SDK methods directly. Generated help and skills/sandbox/SKILL.md include the new forms.

This PR builds on vercel/sandbox#391 and should merge after it. Public guidance is in vercel/front#91668; that docs PR remains gated on stable CLI publication.

Verification: 231 CLI tests passed across 29 files, typecheck and build passed, and generated help and diff checks passed. A live playground probe verified one-command stdin through sandbox exec and sandbox users exec, then removed the green throwaway sandbox. An earlier separate-process probe wrote two stdin chunks, closed input, read stdout and stderr separately, resolved a published port, and verified user/group membership changes. Independent read-only reviews found no P0/P1/P2 issues.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated
sandbox	Ready	Preview, v0	Oct 17, 2026 12:17am UTC
sandbox-cli	Ready	Preview, v0	Oct 17, 2026 12:17am UTC
sandbox-sdk-ai-example	Ready	Preview, v0	Oct 17, 2026 12:17am UTC
workflow-code-runner	Ready	Preview, v0	Oct 17, 2026 12:17am UTC