docs: design cross-device Team completion (#1566)

## Description

Defines the three-layer design for coordinator-owned cross-device legacy Team completion. It documents first-valid-finish-wins semantics, atomic local application, safe retry behavior, compatibility, and validation requirements.

## Type of Change

- [ ] 🚀 Feature (new functionality)
- [ ] 🐛 Bug fix (fixes an issue)
- [x] 📚 Documentation (docs-only change)
- [ ] 🔧 Maintenance (refactor, chore, CI, etc.)
- [ ] 🧪 Testing (test-only changes)

## Testing

- [x] Relevant checks pass locally (`pnpm run tsc`, `pnpm run lint`, `pnpm run test`)
- [ ] Added/updated tests for changes
- [x] Manually verified changes work as expected

## Checklist

- [x] Code follows project style (`pnpm run lint` passes for touched files)
- [x] Self-review completed
- [x] Documentation updated (if needed)
- [x] No new warnings introduced