Use proper 'duplicate' state_reason for issue closures

- Update auto-close script to use state_reason: 'duplicate' instead of 'not_planned'
- Simplify workflow detection logic to only check for duplicate state_reason
- Remove fallback logic for backward compatibility - use modern GitHub API

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

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

scripts/auto-close-duplicates.ts

@@ -65,7 +65,7 @@ async function closeIssueAsDuplicate(
     'PATCH',
     {
       state: 'closed',
-      state_reason: 'not_planned',
+      state_reason: 'duplicate',
       labels: ['duplicate']
     }
   );