Add package provenance and snapshot metadata

[dlebauer]

Follow-up from review of #12.

Goal

Add clear provenance metadata for each packaged data snapshot so users can tell what was exported, when it was built, and what policy assumptions apply.

Track

• snapshot or export date for the source data
• package build date
• source provenance for the export
• public-data policy / filtering assumptions included in the package
• where this metadata should be exposed to users (for example an exported object, installed metadata file, or helper)

Acceptance criteria

• Users can discover snapshot provenance from the installed package.
• The build process defines where this metadata comes from and how it is refreshed.
• Documentation points users to the provenance metadata.

[dlebauer]

Closing as low priority; can be reopened in future