Document Gradle and Paper workflow

CONTRIBUTING.md

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+# Contributing
+
+## Development Setup
+
+Use Java 25 or newer. The Gradle wrapper downloads Gradle 9.8; dependencies are declared in `build.gradle` and resolved from Maven Central, PaperMC, and EngineHub.
+
+```sh
+./gradlew build
+```
+
+The standard build runs JUnit 5 tests without starting Minecraft. Tests use Mockito for Bukkit and external-plugin boundaries. Keep domain and configuration behavior testable without a running server.
+
+## Live Server Tests
+
+Live tests are opt-in and run separately from `build`:
+
+```sh
+METROPOLIS_RCON_HOST=127.0.0.1 \
+METROPOLIS_RCON_PORT=25575 \
+METROPOLIS_RCON_PASSWORD=... \
+METROPOLIS_SERVER_DIR=/path/to/server \
+./gradlew integrationTest
+```
+
+Use a disposable or backed-up Paper server with compatible WorldGuard and WorldEdit plugin jars installed. Keep RCON local, use a temporary password, and restore the server after tests that modify regions or worlds.
+
+## Pull Requests
+
+- Keep changes focused and include tests for behavior changes.
+- Run `./gradlew build` before submitting.
+- Run `integrationTest` only when changing behavior that requires Paper or the external plugins; do not add it to the default build.
+- Update `README.md` and project descriptions when user-facing setup, commands, or dependencies change.
+- Keep generated build output, server data, credentials, and local `.env` files out of commits.