Host desktop control with required visual verification

What this changes for operators

• Operators evaluating Agent Zero for host control must decide whether computer_use_remote is allowed at all on the host — the default trust mode is opt-in but the runtime checks are enforceable.
• Workcell operators should know that screenshot capture is now ephemeral and context-scoped by default; auditing what the agent saw requires explicit durable capture.
• Operators using the existing linux-desktop skill: verify your skill routes to the path you expect; host and container desktops are now cleanly separated.