fix(scripts): reject self-referential duplicates

[RerankerGuo]

Summary

Prevent comment-on-duplicates.sh from proposing the triggering issue as a duplicate of itself.

The script previously accepted identical base and duplicate issue numbers, posted a self-referential duplicate comment, and returned success. That comment could then be consumed by the automatic duplicate closer.

Changes

• Reject a duplicate candidate whose number equals BASE_ISSUE.
• Fail before making any GitHub API calls.
• Keep the existing numeric and issue-existence validation unchanged.

Verification

• Reproduced the original behavior with BASE_ISSUE=123 and duplicate 123: the script posted the comment and exited 0.
• Re-ran the same fixture after the change: the script reports that issue #123 cannot duplicate itself, exits 1, and makes no gh calls.
• Verified a normal 123 -> 456 case still validates both issues and posts the expected comment.
• Ran bash -n scripts/comment-on-duplicates.sh.
• Ran git diff --check.

Scope

This PR only rejects self-references. It does not change duplicate ranking, comment wording, or automatic-close timing.