[WIP] Enhance MCTS hybrid search workflows and build system

[Copilot]

• Improve comment in src/gpu/nnue_weight_accessor.h:248 to explain why stderr is used instead of stdout
• Document memory leak and cleanup mechanism in src/gpu/metal/metal_backend.mm:233-235
• Verify cleanup_parallel_hybrid_search() declaration in src/main.cpp (already declared in uci/uci.h)
• Fix potential crash in src/mcts/ab_integration.cpp:1129-1132 by returning fallback instance instead of throwing
• Add CodeQL build artifacts to .gitignore to prevent accidental commits

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[NripeshN]

@copilot try again