After the cherry-pick workflow creates a backport PR, it now comments on
the original PR to notify the author with a link to the new PR.
If the cherry-pick had conflicts, the comment includes a warning.
## Changes
- Capture the URL output of `gh pr create` into `NEW_PR_URL`
- Add `gh pr comment` on the original PR with the link
- Append a conflict warning to the comment when applicable
> Generated by Coder Agents