docs: add AGENTS.md with custom-image dependency install rule

.opencode/agents/architect.md

@@ -9,6 +9,8 @@ You are a system architect for the FastSync project — a high-performance file
 
 Make high-level design decisions. Evaluate trade-offs, plan module interactions, design data flow, and ensure architectural coherence across the codebase.
 
+> **Environment rule:** dependency installation must always use the project's custom Docker image (repo-root `Dockerfile`, same as CI) — never ad-hoc host package installs. See `AGENTS.md`.
+
 ## Project Architecture
 
 ### Module Map

.opencode/agents/cmake-expert.md

@@ -69,6 +69,7 @@ tests/         — test sources (globbed as TEST_SRCS)
 - Include directories: `src/shared`, `src/server`, `src/client`, `tests` (for test target).
 - Sanitizer support is commented out but present (`-fsanitize=address`).
 - Build with `cmake -B build -S . && cmake --build build -j$(nproc)`.
+- Install dependencies only via the project's custom Docker image (repo-root `Dockerfile`, same image CI uses) — never via host package installs; see `AGENTS.md`.
 
 ## When Making Changes
 

AGENTS.md

@@ -0,0 +1,33 @@
+# AGENTS.md
+
+FastSync is a high-performance file synchronization system written in C11. It supports TCP and SSH transports, TLS encryption (OpenSSL), streaming zstd compression, multithreaded transfers, and incremental sync. The build uses CMake; CI runs on Gitea Actions (`.gitea/workflows/ci.yaml`).
+
+## Dependency installation
+
+**Rule: always install dependencies using the project's custom Docker image — never via ad-hoc system package installs on the host** (no `apt-get install` / `pip install` on the host machine).
+
+The image is built from the repo-root `Dockerfile` and is the same image CI uses: `gitea.tap-tap.win/taptap/fastsync-ci:v7`. It contains the full toolchain: gcc/g++, CMake, libzstd-dev, libssl-dev, make, git, cppcheck, clang-format, python3 + pytest, openssh-client, and Node.js.
+
+```bash
+# Build the image from the repo-root Dockerfile
+docker build -t fastsync-ci:local .
+
+# Provision dependencies and build inside the container (repo mounted at /workspace)
+docker run --rm -v "$PWD:/workspace" -w /workspace fastsync-ci:local \
+  sh -c 'cmake -B build -S . && cmake --build build -j$(nproc)'
+```
+
+If a dependency is missing from the image, add it to the `Dockerfile` (and rebuild) rather than installing it on the host.
+
+## Build
+
+```bash
+cmake -B build -S . && cmake --build build -j$(nproc)
+```
+
+## Test
+
+```bash
+./build/tests                # unit tests
+python3 -m pytest tests/     # integration tests
+```