Add ona login prerequisite to README

Co-authored-by: Ona <no-reply@ona.com>

Author: meysholdt

README.md

@@ -76,6 +76,14 @@ Copy the two automation files into your repo:
 
 Adjust the agent prompts if your project uses a different build tool (e.g., replace `./mvnw` with `./gradlew` or `npm`).
 
+#### Prerequisites
+
+Log in to Ona before running any `ona ai` commands:
+
+```bash
+ona login
+```
+
 #### Install automations
 
 Use the Ona CLI to register each automation: