Add CODEOWNERS file

## Summary
Define code owners so PRs auto-request review from the right people.

## Files
- new `.github/CODEOWNERS`

## Acceptance criteria
- [ ] `/contracts/` → @Miracle656
- [ ] `/sdk/` → @Miracle656
- [ ] `/frontend/wallet/` → @Miracle656
- [ ] `/packages/agent/` → @Miracle656
- [ ] `/.github/` → @Miracle656