[codex] add minimize-only window rule to computer skill

[quanru]

Summary

• add a critical safety rule to the Computer skill: minimize windows instead of closing them unless the user explicitly asks to close them
• update the UI-element specificity example to use the minimize button so the guidance stays internally consistent

Why

Closing a window during desktop automation can destroy user state and go beyond the requested action when the actual need is only to get the window out of the way.

Impact

This makes Computer skill automation safer and reduces accidental app or window termination.

Validation

• git diff --check