Add sandbox command

[gtsiolis]

Add lstk sandbox command group for managing cloud-hosted LocalStack sandbox instances.

Command	Description
lstk sandbox create <name>	Create a sandbox instance
	--timeout 60 — Instance lifetime in minutes
	-e KEY=VALUE — Environment variable to pass to the instance
lstk sandbox list	List instances in a table
lstk sandbox describe <name>	Show raw JSON instance state
lstk sandbox delete <name>	Delete an instance
	--wait — Poll until fully deleted
	--wait-timeout 5m — Max time to wait when --wait is set
lstk sandbox logs <name>	Fetch instance logs
lstk sandbox url <name>	Print endpoint URL (scripting-friendly)
lstk sandbox reset <name>	Reset LocalStack state via /_localstack/state/reset

Sandbox name is a positional argument, a hidden --name flag exists for migration compatibility from CLIv1.