fix: enable CodeRabbit auto-review on alpha branch (#1235)

<!-- acp:session_id=session-e0d41db6-be71-4a13-86f3-af497b4f3e80
source=#1134 last_action=2026-04-07T13:38:00Z retry_count=0 -->
## Summary

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

Closes #1134

## Test plan

- [ ] Create a test PR targeting `alpha` and verify CodeRabbit triggers
a review
- [ ] Confirm PRs targeting `main` continue to receive CodeRabbit
reviews


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Co-authored-by: Ambient Code Bot <bot@ambient-code.local>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

.coderabbit.yaml

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