Add repository AGENTS guide

AGENTS.md

@@ -0,0 +1,16 @@
+# AGENTS.md
+
+This repository is `opentelemetry-java`, the core Java API and SDK implementation for OpenTelemetry.
+
+Primary contributor guidance lives in [CONTRIBUTING.md](CONTRIBUTING.md). Read that first and treat
+it as the source of truth for repository layout, build and test commands, style expectations, and
+scope.
+
+Additional guidance for agents:
+
+* Prefer small, localized changes over broad refactors.
+* Read the target module's `build.gradle.kts` and nearby package structure before editing.
+* Match existing package naming, visibility, and module boundaries.
+* When changing public API, check for ripple effects across `all`, BOM, incubator, exporters,
+  shims, and integration tests.
+* Mention generated outputs such as `docs/apidiffs/current_vs_latest` when they are affected.