Persist explicit relations and reconcile published Markdown references #89

Work

Implement relations using the shared contract and extract references through #86. Update derived edges after successful authorized task saves or publication of memory revisions. Keep explicit links separate from derived links; support idempotent reconciliation and a backfill dry run.

Acceptance

• Adding/removing Markdown references updates only corresponding derived edges.
• Pending/rejected proposals cannot change published backlinks.
• Transaction failure, revision race and retry cannot leave partial or duplicate relations.
• Archive, expiry, restore and permission changes follow the lifecycle contract.

Dependencies

#80, #81, #86 and the relation contract under #75.