refactor(ui): model Team setup as reducer state (#1553)

## Description
Introduces one discriminated `SetupSessionState` and exhaustive reducer for Team setup lifecycle, commands, scoped errors, navigation, and completion. Stale async outcomes are discarded by generation and server action gates are enforced before effects are queued.

Part 2 of codemem-752i.4.

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

## Testing
- [x] Direct reducer transition, stale-generation, gate, retry, and completion tests
- [x] `pnpm run tsc`
- [x] `pnpm run lint`

## Checklist
- [x] Code follows project style
- [x] Self-review completed
- [x] No new warnings introduced