Switch auto-merge to merge commits (#15)

Auto-merge workflow was using squash merges; change it to use standard
merge commits.

- **`auto-merge.yaml`**: Set `merge_method: 'merge'` (was `'squash'`)

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.github/workflows/auto-merge.yaml

@@ -92,7 +92,7 @@ jobs:
                 owner: context.repo.owner,
                 repo: context.repo.repo,
                 pull_number: prNumber,
-                merge_method: 'squash',
+                merge_method: 'merge',
               });
               console.log(`PR #${prNumber} merged successfully`);
             } catch (error) {