Add AGENTS.md

[Copilot]

Adds AGENTS.md to give AI coding agents (and new contributors) a concise orientation to the repository.

Contents

• Structure — maps Sources/CSFBAudioEngine/ subdirectories to their roles (decoders, encoders, player, metadata, I/O, etc.)
• Build & test — swift build / swift test; notes macOS + Xcode requirement and binary XCFramework dependency resolution via SPM
• Code style — clang-format (LLVM-based, 120-col, 4-space indent) for C/ObjC/C++; .editorconfig baseline; Swift style by convention
• Language standards — C11, C++20
• Design patterns — decoder/encoder protocol hierarchy (SFBAudioDecoding, SFBPCMDecoding, SFBDSDDecoding, SFBPCMEncoding), SFBInputSource/SFBOutputTarget abstraction, SFBAudioPlayer on AVAudioEngine
• Adding decoders/encoders — where to place files, which protocol to implement, registration pattern, license compatibility check
• LGPL note — libsndfile, mpg123, libtta-cpp, LAME, and Musepack encoder must remain dynamically linked