Fix duplicate pollution: filter closed issues from detection

- Closed issues no longer appear in duplicate scans
- Prevents infinite reappearance of merged duplicates
- Deleted 29 low-value closed issues to shrink DB
- Closes bd-c577

Amp-Thread-ID: https://ampcode.com/threads/T-51165824-9559-4696-919f-2c40459d5ef9
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/duplicates.go

@@ -49,8 +49,16 @@ Example:
 		os.Exit(1)
 		}
 
-		// Find duplicates
+		// Filter out closed issues - they're done, no point detecting duplicates
-		duplicateGroups := findDuplicateGroups(allIssues)
+		openIssues := make([]*types.Issue, 0, len(allIssues))
+	for _, issue := range allIssues {
+		if issue.Status != types.StatusClosed {
+			openIssues = append(openIssues, issue)
+		}
+	}
+
+	// Find duplicates (only among open issues)
+	duplicateGroups := findDuplicateGroups(openIssues)
 
 		if len(duplicateGroups) == 0 {
 			if !jsonOutput {