chore: add AGENTS.md with Cursor Cloud dev environment instructions

AGENTS.md

@@ -0,0 +1,26 @@
+## Cursor Cloud specific instructions
+
+This is the **Trolley Java SDK** — a pure Java HTTP client library for the Trolley payments API. There are no local services, databases, or containers to run.
+
+### Build & test commands
+
+- **Compile:** `mvn compile -B`
+- **Package (skip tests + GPG):** `mvn package -DskipTests -Dgpg.skip=true -B`
+- **Run tests:** `mvn test -B` (requires valid API credentials; see below)
+
+Set `JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64` before running Maven commands — the javadoc plugin requires it.
+
+### Integration tests & credentials
+
+All tests under `src/test/java/com/trolley/integration/` are **live integration tests** that call the Trolley API at `api.trolley.com`. They require a `.env` file in the repo root with:
+
+```
+ACCESS_KEY=<your-trolley-access-key>
+SECRET_KEY=<your-trolley-secret-key>
+```
+
+Without valid credentials, all 18 tests will fail with `AuthenticationException`. This is expected behavior — there are no unit tests or mocked tests in this repository.
+
+### JVM config
+
+The `.mvn/jvm.config` file contains `--add-opens` flags required for PowerMock compatibility with modern JDKs. These are picked up automatically by Maven.