refactor: consolidate performance configuration into single source of truth

- Create centralized PERFORMANCE_DEFAULTS constants
- Add comprehensive PerformanceConfig interface with all settings
- Implement environment variable parsing with type safety
- Add configuration validation with detailed error messages
- Add getOptimizedConfig() for auto-tuning based on system resources
- Add getConfigSummary() for human-readable configuration display
- Remove duplicate configuration in agent-optimized.ts

Author: DaMandal0rian