Dedupe automation should skip pull requests returned by issues API

Summary

The dedupe automation uses the /issues API, which returns pull requests alongside issues. The scripts currently do not filter PR items.

Steps to Reproduce

1. Run scripts/auto-close-duplicates.ts or scripts/backfill-duplicate-comments.ts.
2. Ensure at least one pull request exists in the issues list (GitHub includes PRs with a pull_request field).

Expected Behavior

PR items are ignored and never processed by the dedupe scripts.

Actual Behavior

PR items can be processed, which may close or comment on PRs incorrectly.

Proposed Fix

Skip any list items that include pull_request metadata before processing.