Add multi-provider code review to GitHub Action

[keithah]

Summary

• Run parallel reviews with multiple free opencode providers
• Synthesize results into comprehensive code review
• Add configurable provider list via REVIEW_PROVIDERS env var
• Include error handling and timeout management
• Add documentation for the feature

Changes

• Created github/multi-review.ts script to handle parallel reviews
• Modified .github/workflows/review.yml to use multiple providers
• Added github/MULTI_REVIEW.md documentation

Providers Used

• opencode/big-pickle - Large reasoning model
• opencode/grok-code - Code-specialized model
• opencode/minimax-m2.1-free - Free tier model
• opencode/glm-4.7-free - Free GLM model

Benefits

• Comprehensive Reviews: Multiple perspectives catch different issues
• Redundancy: If one provider fails, others continue
• Cost Efficiency: Uses free providers
• Quality Synthesis: Combines the best insights from all reviews