docs: add repo instructions

AGENTS.md

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+# AGENTS.md
+
+## Repository Role
+
+This repository contains the C++ server source and build system for:
+
+- `game`
+- `db`
+- `qc`
+- smoke/login test binaries
+
+The current production VPS runs a Debian deployment built from `main`.
+
+## Working Rules
+
+- Prefer small, reviewable changes directly on `main`.
+- Keep Debian runtime stability ahead of broad refactors.
+- Do not commit `build/`, temporary captures, or host-local debug output.
+- Do not commit secrets or production-only credentials.
+
+## Build And Test
+
+Typical local build:
+
+```bash
+cmake -S . -B build -G Ninja
+cmake --build build
+ctest --test-dir build --output-on-failure
+```
+
+Important test targets:
+
+- `metin_smoke_tests`
+- `metin_login_smoke`
+
+## High-Risk Areas
+
+Changes in these areas require extra care:
+
+- auth/login flow
+- packet headers and packet structs
+- `SecureCipher`
+- `fdwatch` and socket output path
+- DB login/account flow
+
+When changing login, auth, packet, or network behavior:
+
+- build and run smoke tests
+- keep protocol compatibility with `m2dev-client-src`
+- keep client asset behavior in sync with `m2dev-client`
+
+## Production Verification
+
+The current VPS has a root-only end-to-end login check:
+
+```bash
+/usr/local/sbin/metin-login-healthcheck
+```
+
+Use it after risky auth/network/runtime changes.
+
+## Cross-Repo Boundaries
+
+- server code changes belong here
+- runtime/config/systemd/docs changes belong in `m2dev-server`
+- client protocol implementation changes belong in `m2dev-client-src`
+- client asset/login screen/server list changes belong in `m2dev-client`