Define typed item relations and their lifecycle #88
Description
Work
Design a small SQL relation model using canonical endpoints from #73. Define direction/cardinality for references, related, implements and supersedes; coordinate blocked_by/discovered_from with #76. Record provenance (explicit versus Markdown-derived), author and relevant source revision.
Acceptance
- Direction, allowed endpoint types, duplicate/self-link handling and removal rules documented.
- Supersedes cannot cycle; related is symmetric without duplicate pairs.
- Private, cross-project, archived/expired and pending-revision cases specified.
- No generic graph database or automatic extraction dependency introduced.
Dependencies
#79 reference contract. Deliver a shared relation contract before graph/dependency persistence.