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>

modules/setting/repository.go

@@ -89,6 +89,7 @@ var (
 			TestConflictingPatchesWithGitApply       bool
 			RetargetChildrenOnMerge                  bool
 			DelayCheckForInactiveDays                int
+			DefaultDeleteBranchAfterMerge            bool
 		} `ini:"repository.pull-request"`
 
 		// Issue Setting
@@ -213,6 +214,7 @@ var (
 			TestConflictingPatchesWithGitApply       bool
 			RetargetChildrenOnMerge                  bool
 			DelayCheckForInactiveDays                int
+			DefaultDeleteBranchAfterMerge            bool
 		}{
 			WorkInProgressPrefixes: []string{"WIP:", "[WIP]"},
 			// Same as GitHub. See