Optional fields should use Effect Option type #61

Problem

File: src/parallel/types.ts

Optional fields use TypeScript optional syntax instead of Effect's Option type:

• Line 3: mode?: "one-shot" | "agentic" - Could be Option<...>
• Line 4: objective?: string - Should be Option<string>
• Line 30: objective?: string - Same issue

Proposed Fix

For consistency with Effect TS patterns, consider using Option<T> for optional fields in domain types.