Add AGENTS guidance

Author: adriaandotcom

AGENTS.md

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+# Agent Instructions
+
+This repository contains the public scripts for Simple Analytics. When making changes keep the following points in mind.
+
+## Development
+
+- Use **Node.js 22.16** (see `.nvmrc`).
+- Run `npm run build` to compile the scripts. This generates the files in `dist/` which should be committed.
+- Format code with Prettier before committing. You can run `npx prettier -w .`.
+- Execute `npm run test:unit` to run the unit tests.
+
+## References
+
+Most work happens in `src/` with `compile.js` creating the distributable versions. Read `README.md` for more details about contributing and running scripts locally.