chore: complete Epic 0 retrospective and update sprint status

- Add Epic 0 retrospective document with lessons learned, action items, and Epic 1 preparation plan
- Update sprint-status.yaml: epic-0 → done, epic-0-retrospective → done

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

_bmad-output/implementation-artifacts/sprint-status.yaml

@@ -41,13 +41,13 @@ story_location: "_bmad-output/implementation-artifacts"
 
 development_status:
   # Epic 0: Foundation Migration & Infrastructure Setup
-  epic-0: in-progress
+  epic-0: done
   0-1-rename-folders-to-declarations-in-database: done
   0-2-rename-folders-to-declarations-in-backend: done
   0-3-rename-folders-to-declarations-in-frontend: done
   0-4-configure-redis-for-cache-queue-and-sessions: done
   0-5-add-foundation-database-migrations-and-declaration-status-flow: done
-  epic-0-retrospective: optional
+  epic-0-retrospective: done
 
   # Epic 1: Team Management & Permission System
   epic-1: backlog