fix: bd doctor no longer warns about deletions.jsonl

The multiple JSONL files check was incorrectly flagging deletions.jsonl
as a problem, even though it is a valid system file for tracking deleted
issues. Added deletions.jsonl to the skip list alongside backups and
merge artifacts.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/doctor/legacy.go

@@ -132,13 +132,14 @@ func CheckLegacyJSONLFilename(repoPath string) DoctorCheck {
 			continue
 		}
 
-		// Skip merge artifacts and backups
+		// Skip merge artifacts, backups, and system files
 		lowerName := strings.ToLower(name)
 		if strings.Contains(lowerName, "backup") ||
 			strings.Contains(lowerName, ".orig") ||
 			strings.Contains(lowerName, ".bak") ||
 			strings.Contains(lowerName, "~") ||
-			strings.HasPrefix(lowerName, "backup_") {
+			strings.HasPrefix(lowerName, "backup_") ||
+			name == "deletions.jsonl" {
 			continue
 		}