fs/session: statewait must use h.ag.WaitChange, not Core.WaitChange

Session.WaitChange delegates to a.Agent() (the active agent),
so statewait was blocking on the WRONG agent's state changes.
Agent.WaitChange uses per-agent changeCond, so it properly
blocks until this specific agent's state actually changes.

fs/session/files.go

@@ -367,7 +367,7 @@ func (h *agentHelper) fileSpec(name string, mode os.FileMode) FileSpec {
 			if base == "" {
 				base = h.ag.State()
 			}
-			v, ok := h.sess.Core.WaitChange(ctx, agent.WatchState, base)
+			v, ok := h.ag.WaitChange(ctx, agent.WatchState, base)
 			if !ok {
 				s := h.ag.State()
 				return []byte(s + "\n"), s, nil