Test: Explore pre-script fetch makes data available at runtime

This issue is a dedicated Full Send proof run for the Explore agent.

The test demonstrates:
1. A trusted pre-script fetches public data before the sandbox starts.
2. Full Send mounts that fetched file into the agent sandbox.
3. Explore reads the file at runtime and reports the fetched value.

To run the proof, add the comment `/prefetch-demo`.