IDLIB, Syria — A suicide bomber set off his explosives late on Thursday in northwestern Syria, killing the co-founder of the country’s main al-Qaida-linked group that controls much of the northwest, a war monitor said. Al-Qahtani co-founded the Nusra Front in Syria, a militant group that later renamed itself Hayat Tahrir al-Sham and claimed it had severed ties with al-Qaida. …