Create root AGENTS.md for core development

The AGENTS.md file in the project root is meant for development of
`laravel-mongodb` itself.

Author: paulinevos

AGENTS.md

@@ -0,0 +1,11 @@
+# Contributing to `mongodb/laravel-mongodb`
+
+Guidance for agents working on the development of this library. For assisting
+end users who *use* the package in their own applications, see the
+[`laravel-mongodb` skill](skills/laravel-mongodb/SKILL.md).
+
+## Keeping the skill in sync
+
+When a change adds, removes, or alters user-facing behaviour covered by the
+[`laravel-mongodb` skill](skills/laravel-mongodb/SKILL.md), update the relevant
+skill reference file in the same PR so the skill stays accurate.