docs: cross-repo DRY audit + end-to-end migration roadmap
- CROSS_REPO_DRY_AUDIT.md: identified 14 duplication patterns across
9 product backends + web clients (~4,200 LOC duplicated)
- CROSS_REPO_DRY_MIGRATION_ROADMAP.md: 5-phase execution plan with
concrete file lists, migration steps, verification checkpoints,
rollback strategy, and success metrics
- Phase 0: quick wins (delete errors.ts re-exports, standardize product-config)
- Phase 1: @bytelyst/fastify-auth (auth + request-context for 9 repos)
- Phase 2: @bytelyst/backend-config + datastore migration for 3 older repos
- Phase 3: backend-flags, backend-telemetry, domain-events (FlowMonk + ActionTrail)
- Phase 4: web client DRY (telemetry, diagnostics, product-config conveniences)
- Estimated: 11 days, 5 new packages, ~3,260 LOC eliminated
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