Clear stale runner logs before starting ATVM watcher

- update the watcher start helper to remove the old /tmp/<build-name>.log file before starting a fresh watcher instance for a reused build name
- prevent the watcher from inheriting stale started_at timestamps from previous runs with the same build name and immediately exiting as HUNG
- make reused-build watcher startup safer by resetting both watcher state and the matching temporary runner log

atvm/watcher-service/start-atvm-run-watcher.sh

@@ -49,8 +49,10 @@ if [[ -z "$BUILD_NAME" ]]; then
 fi
 
 RUN_DIR="${STATE_ROOT}/${BUILD_NAME}"
+RUN_LOG="/tmp/${BUILD_NAME}.log"
 systemctl stop "atvm-run-watcher@${BUILD_NAME}.service" >/dev/null 2>&1 || true
 rm -rf "$RUN_DIR"
+rm -f "$RUN_LOG"
 mkdir -p "$RUN_DIR"
 
 cat >"${RUN_DIR}/watch.env" <<EOF