Heartbeat: re-arm CI watch on upstream push instead of going idle

When the heartbeat detects a new upstream commit on a previously watched open PR (e.g. Slava pushed to PR #15083), update active-ci-watch.env to STATUS=watching with the new HEAD_SHA rather than clearing to idle. This keeps CI monitoring alive across author pushes. Update verify-pr-status.sh or heartbeat step 0 as needed.