SHUJA’IYYA, Gaza Strip — Silence is the most startling sound in this town. A month ago this region was home to some of the fiercest fighting in the two-year war in the Gaza Strip . But when NBC News visited an Israeli military outpost on Wednesday, the area appeared completely leveled and entirely devoid of people. The soldiers manning the outpost are overseeing a delicate aspect of the now three-week-old ceasefire: the “yellow line,” a boundary of yellow blocks and flags laid out by the Israel