feat(explainability): add diagnostics JSON, per-memory explain, lifecycle trace

Phase 4 Tasks 4.1-4.3:
- memory-diag health --json: machine-readable MemoryDiagJSON output
- memory-diag explain: per-memory render status with strength, reasons,
  evidence event IDs
- memory-diag trace --memory <id>: lifecycle history from evidence events
  and relations (superseded_by, reinforced_by)
- MemoryRenderStatus type with 9 statuses
- All diagnostics are read-only, no storage mutations
- Privacy-safe: redacted text previews, no raw secrets
- 270 tests pass, typecheck pass

Author: sdwolf4103