docs: add testing guidelines to AGENTS.md

Add reminders to write tests for new features/bugs and run tests before committing.

Author: Rinse12

AGENTS.md

@@ -2,4 +2,6 @@
 - Make sure to use the right node version, run "nvm use"
 - you need to make sure building passes when you make a change, run "yarn build && yarn build:test"
 - We use yarn in this repo, not npm
-- do not run "yarn install" until you run "nvm use" first and make sure we're on the right node version
+- do not run "yarn install" until you run "nvm use" first and make sure we're on the right node version
+- When you add a feature or fix a bug, add a test for it.
+- Before you commit a change make sure all tests pass