feat: Add artifact system and manager view for Code Review Agent

Implements Google Antigravity-inspired improvements:
- Artifact system with structured outputs (plan, summary, fixes, visuals)
- CodeReviewAgentManager for parallel execution
- Enhanced prompts with strategic planning phase
- Comprehensive tests and documentation

Co-authored-by: phodal <472311+phodal@users.noreply.github.com>

Author: Copilot