Generate stage thumbnails and checksums from the CLI

[Swarek]

Problem

A new stage pack requires authors to manually discover thumbnail and checksum conventions from an existing stage.

Desired behavior

Provide a targeted command such as npm run stage:assets -- <id> that generates deterministic derived assets.

Acceptance criteria

• Generate preview.thumb.webp from the stage preview.
• Regenerate SHA256SUMS for distributed stage assets in a stable order.
• Never overwrite prose provenance or an upstream license file.
• Report missing required source assets with actionable messages.
• Add tests for deterministic output and path safety.
• Document the command in stages/README.md.