mgr: KillSession uses context cancellation, not direct Interrupt

Session death propagates via context (SIGKILL semantics).
User interrupt (Ctrl-C) is a separate signal via agent.Interrupt().

mgr/persist.go

@@ -311,7 +311,7 @@ func (s *Manager) KillSession(id string) {
 	delete(s.sessions, id)
 	s.mu.Unlock()
 	if sess != nil {
-		sess.Cancel()
+		sess.Cancel() // signal: context cancellation propagates to all agents
 		sess.Core.Close()
 		s.removePersistedAgent(id)
 		s.cfg.Log.Info("killed session %s", id)