Interface LinkedPR

May be null for deleted GitHub accounts ("ghost" users); the declared type on GitHub's API allows null here even though the REST schema example typically shows a populated user.

ISO timestamp of the linked PR's last update, surfaced from scout's timeline-event metadata when available (#97). Optional so existing callers and fixtures that don't carry the field continue to type-check. Used by isLinkedPRStalled to flag open PRs that haven't been touched in the last STALLED_PR_THRESHOLD_DAYS days.