Add AGENTS.md for OpenAI codex agent.

Author: winlinvip

AGENTS.md

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+# Contributor Guide
+
+This is the guide for the OpenAI Codex agent.
+
+## General repository layout
+- Source code: trunk/src/ contains the C++ code for the SRS server.
+  - trunk/src/core and trunk/src/kernel contain the common definitions.
+  - trunk/src/protocol contains the media streaming protocol implementations.
+  - trunk/src/app contains the application-level implementations.
+  - trunk/src/main contains the main entry points for the programs.
+- Source code: proxy/ contains the proxy server in Go for RTMP/SRT/WebRTC proxying.
+- UTests: trunk/src/utest contains the unit tests using gtest.
+- Third-party dependencies: trunk/3rdparty contains the dependency libraries. You may refer to these codes, but never attempt to change or update them.
+  - trunk/3rdparty/st-srs is the coroutine library (state-threads).
+  - trunk/3rdparty/srs-bench is the integration test tool for blackbox tests.
+  - trunk/3rdparty/gtest-fit is the gtest framework used for unit tests.
+  - trunk/3rdparty/ffmpeg-4-fit is the codec library for transcoding audio streams such as AAC with Opus.
+  - trunk/3rdparty/openssl-1.1-fit is used for RTMP handshake and WebRTC DTLS handshake.
+  - trunk/3rdparty/libsrtp-2-fit is used for SRTP in WebRTC.
+  - trunk/3rdparty/srt-1-fit is the SRT implementation.