StuLabAgent

fix(agent,teacher,common): async cmd dispatch, errno import, atomic yaml writes, treeview diff refresh, recv body deadline, subprocess timeouts, hostname validation

fix(common,agent): unify frame size limit, quote YAML special chars, lock config removal in disable_restrictions

docs: reorganize environment requirements section in README

refactor(agent,teacher): extract shared utils to common/utils; remove wincli_escape

add: QQ group QRcode