Computer-use screenshots now persist to durable chat-scoped storage by default

What this changes for operators

• Reverses the prior ephemeral-by-default posture for computer-use screenshots, so operators who relied on screenshots being transient must now account for retained artifacts
• Changes deployment storage characteristics: long-running computer-use sessions accumulate screenshots in chat context, requiring storage planning and retention/cleanup review
• Directly hits the Grid/workcell calibration concern of persistence and cleanup for real computer access