BAGHDAD, Iraq — Talks aimed at persuading the rebel Shiite cleric Muqtada al-Sadr to withdraw from Najaf collapsed yesterday, paving the way for a resumption of the bloody battle for control of the city. Iraq's national-security adviser, Muwaffak al-Rubaie, said he was "overcome with deep sorrow" by the failure of the negotiations, launched late last week in a bid to avert a potentially explosive showdown between al-Sadr's black-clad militia fighters and the joint U.S.-Iraqi force ringing the ancient city.