Add AGENTS.md with Cursor Cloud development instructions

AGENTS.md

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+# BetterHUD - Agent Instructions
+
+## Cursor Cloud specific instructions
+
+### Project overview
+
+BetterHUD is a client-side Minecraft Fabric mod that adds a customizable HUD overlay. It is a single-module Gradle project (not a monorepo) with no external services, databases, or backends.
+
+### Prerequisites
+
+- **JDK 25** is required (`sourceCompatibility = JavaVersion.VERSION_25` in `build.gradle`). The update script installs `openjdk-25-jdk` via apt.
+- Set `JAVA_HOME=/usr/lib/jvm/java-25-openjdk-amd64` before running Gradle commands.
+
+### Common commands
+
+| Task | Command |
+|---|---|
+| Build | `JAVA_HOME=/usr/lib/jvm/java-25-openjdk-amd64 ./gradlew build` |
+| Check (lint + tests) | `JAVA_HOME=/usr/lib/jvm/java-25-openjdk-amd64 ./gradlew check` |
+| Run Minecraft client | `JAVA_HOME=/usr/lib/jvm/java-25-openjdk-amd64 ./gradlew runClient` |
+| Clean | `JAVA_HOME=/usr/lib/jvm/java-25-openjdk-amd64 ./gradlew clean` |
+
+### Caveats
+
+- There are no test sources in this project (`test NO-SOURCE`); `./gradlew check` still runs `validateAccessWidener` and compilation checks.
+- `./gradlew runClient` launches a full Minecraft client with the mod loaded. On a headless VM it will produce ALSA audio errors (harmless) and requires a display server (Xvfb or similar) to render.
+- The first Gradle invocation downloads Minecraft JARs, mappings, and dependencies (~30s). Subsequent builds are cached and fast (~1-2s).
+- Build output JAR is at `build/libs/betterhud-<version>.jar`.
+- Gradle wrapper (`./gradlew`) auto-downloads Gradle 9.4.1; no manual Gradle install needed.