Snapshot and move what DICE knows

[jimador]

There's no way to move what DICE knows — the store is tied to the process and its backend, with no
portable, self-contained artifact to snapshot it or carry it elsewhere.

Add a local export/import format for knowledge bundles, so a graph — propositions, provenance, and
decay status — can be written out and reloaded faithfully in another instance.

Use cases

• Snapshot a graph to a single portable file for backup.
• Carry knowledge from one DICE instance to another and reload it faithfully — propositions,
  provenance, and decay status intact.
• Share a curated knowledge set without handing someone access to the live store.