cmd/sgai: add CLI action runner

[ucirello]

Summary

• add a CLI action runner path so named workspace actions can execute through the sgai binary without requiring the full interactive server flow
• route built-in and ad-hoc action execution through the new run command and supporting service/config updates, including validation for missing or unknown actions
• cover the runner behavior with focused Go tests and absorb the related go-subprocess-lifecycle skill update into cmd/sgai/skel/.sgai