Creates comprehensive documentation for AI agents working with app/core/:
- Architecture overview with component diagram
- Import patterns (correct enum imports from app.core.enums)
- Core patterns: game creation, action execution, effects, config
- Key classes: GameState, PlayerState, CardDefinition, CardInstance, Zone
- Testing patterns with SeededRandom for determinism
- Security rules for hidden information
- Module independence guidelines for offline fork support
- File organization and quick commands
Updates PROJECT_PLAN.json: 31/32 tasks complete.
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