docs: add repo instructions

AGENTS.md

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+# AGENTS.md
+
+## Repository Role
+
+This repository contains the Windows client source code and build system.
+
+It owns:
+
+- client executable source
+- protocol/client networking code
+- UI/client runtime C++ modules
+- build-time tools such as client-side pack/proto helpers
+
+## Working Rules
+
+- Treat this repository as Windows-first.
+- Prefer protocol-safe, targeted changes over broad refactors.
+- Keep packet and login-flow changes aligned with `m2dev-server-src`.
+- Do not mix packed asset/data changes into this repository unless they are source-side build assets.
+
+## Build Notes
+
+Typical build entrypoint:
+
+```bash
+cmake -S . -B build
+cmake --build build
+```
+
+In practice, most real builds happen on Windows with Visual Studio toolchains.
+
+## High-Risk Areas
+
+- `src/UserInterface/Packet.h`
+- `AccountConnector`
+- `PythonNetworkStream*`
+- `NetStream`
+
+When changing login/auth/protocol behavior:
+
+- mirror the server-side packet expectations in `m2dev-server-src`
+- verify the live asset-side flow in `m2dev-client`
+- avoid silent header drift between client and server
+
+## Cross-Repo Boundaries
+
+- client executable and protocol code belong here
+- packed/unpacked client data under `assets/` belong in `m2dev-client`
+- server packet/auth implementation belongs in `m2dev-server-src`