By Nidal al-Mughrabi and Dan Williams GAZA/JERUSALEM (Reuters) - A truce between Israel and Hamas began on Friday at the hour set by Egyptian mediators, and U.S. President Joe Biden pledged to salve the devastation of the worst fighting in years with humanitarian aid for Palestinians in the Gaza Strip. In the countdown to the 2 a.m. (2300 GMT Thursday) cease-fire, whose timing Hamas had confirmed but Israel did not, Palestinian rocket salvoes continued and Israel carried out at least one air strike. Each side said it stood ready to retaliate for any truce violations by the other. Cairo said it...