Add AGENTS.md with concise repository guidance for AI agents

[reg-pl]

Motivation

• Provide a short, focused, agent-friendly summary of the project so future automated or human agents can quickly understand the engine purpose, layout, build/runtime expectations, configuration model and common controls.

Description

• Add a new AGENTS.md at the repository root that documents a brief project overview, repository layout, build/run expectations (Visual Studio 2022 on Windows, D3D12), runtime/config model (Settings categories and shader locations), documented command-line and keyboard controls, and practical guidance for where agents should make changes.

Testing

• Validated the new file's presence and contents using file-inspection commands (sed, nl, find), and those checks completed successfully.

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Codex Task