enable coderabbit on alpha branch

[ktdreyer]

CodeRabbit is not configured to trigger on PRs that target alpha branch, just main.

Result: we don't get CodeRabbit reviews on these PRs for the new branch.

[Gkrumbach07]

AI triage: Created session triage-gh-1134-coderabbit-alpha to investigate and fix this issue.

[github-actions]

Session session-e0d41db6-be71-4a13-86f3-af497b4f3e80 created (phase: Running). PR will have the ambient-code:managed label.