Auto-close duplicates should preserve existing labels

Summary

The auto-close script sets labels: ['duplicate'], which overwrites existing issue labels.

Steps to Reproduce

1. Run scripts/auto-close-duplicates.ts on an issue with existing labels.
2. The script closes the issue as duplicate.

Expected Behavior

Existing labels remain, and duplicate is added if missing.

Actual Behavior

Existing labels are replaced by duplicate only.

Proposed Fix

Fetch current labels and merge duplicate without overwriting the label set, or use the label-append endpoint.