feat: transactional outbox skill

## Track
Persistence & messaging — see ROADMAP

## Goal
Reliable publish after `repo.save` without putting EventPublisher in the UseCase or repository.

## Acceptance
- [ ] Outbox port + adapter pattern
- [ ] Clear placement vs Handler `entity.commit()`
- [ ] Same-BC listener / bridge integration notes
- [ ] YAGNI: no generic relay framework