Improve AGENTS.md

Author: adriaandotcom

AGENTS.md

@@ -6,8 +6,15 @@ This repository contains the public scripts for Simple Analytics. When making ch
 
 - 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.
+- Format code with Prettier before committing. You can run `npx prettier -w .` or validate with `npm run prettier`.
+
+## Testing
+
+- Execute `npm run test:unit` to run the unit tests
+- When adding a new feature or fixing a bug, please add a test.
+- For every change, run `npm run build` to ensure the test uses the latest compiled version.
+- Ignore most files in the `dist/` directory when checking for diffs, just check `dist/latest/latest.dev.js` and `dist/latest/auto-events.js` (whichever is relevant).
+- Only tests in `test/unit/` are relevant for the AI and agents (it can run without internet access).
 
 ## References