fix: enable CodeRabbit auto-review on alpha branch

Replace the catch-all `.*` regex in base_branches with explicit `main`
and `alpha` entries so CodeRabbit reliably triggers on PRs targeting
either branch.

Closes #1134

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Ambient Code Bot

.coderabbit.yaml

@@ -110,7 +110,8 @@ reviews:
       - "dependabot[bot]"
       - "renovate[bot]"
     base_branches:
-      - ".*"
+      - "main"
+      - "alpha"
 
   # ---------------------------------------------------------------------------
   # Finishing touches