refactor(sdk): improve loadLocalAgents to return validation errors

- Add TypeScript overloads so validate: true returns { agents, validationErrors }
- Add AgentValidationError and LoadLocalAgentsResult types
- Export new types from SDK index
- Move validateAgents to static import (fix dynamic import pattern)
- Add comprehensive tests for loadLocalAgents with type safety
- Fix early return to respect validate flag

Author: brandonkachen