docs: add comprehensive agent orchestration system design

Created detailed design document for Review → Plan → Execute system:
- User experience with GitHub OAuth and dashboard
- Event-driven architecture with LangGraph
- Skills library + MCP tools integration
- Orchestrator model switching (CLI + config)
- Agent capabilities (Backend, Frontend, Tester, DevOps)
- Docker + RunPod deployment strategy
- MVP implementation phases (tonight's target)
- Cost projections and success metrics

Ready for Phase 6: Implementation planning

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: ScientiaCapital