Add DEFAULT_DELETE_BRANCH_AFTER_MERGE setting (#36917)

Add this config option, applying to new repos:

```ini
[repository.pull-request]
DEFAULT_DELETE_BRANCH_AFTER_MERGE = true
```

Defaults to `false`, preserving current behavior.

---------

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>

models/repo/repo_unit.go

@@ -142,6 +142,7 @@ func DefaultPullRequestsConfig() *PullRequestsConfig {
 		AllowRebaseUpdate:          true,
 		DefaultAllowMaintainerEdit: true,
 	}
+	cfg.DefaultDeleteBranchAfterMerge = setting.Repository.PullRequest.DefaultDeleteBranchAfterMerge
 	cfg.DefaultMergeStyle = MergeStyle(setting.Repository.PullRequest.DefaultMergeStyle)
 	cfg.DefaultMergeStyle = util.IfZero(cfg.DefaultMergeStyle, MergeStyleMerge)
 	return cfg