agent: fix missed-wakeup race in WaitChange

notifyChange must hold changeMu when broadcasting. Without the lock,
the broadcast can fire between the consumer's value check and its
cond.Wait call — the signal is lost and the consumer sleeps until the
next change, causing an apparent off-by-one lag.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/agent/core.go

@@ -328,7 +328,9 @@ func (s *agentCore) State() string {
 }
 
 func (s *agentCore) notifyChange() {
+	s.changeMu.Lock()
 	s.changeCond.Broadcast()
+	s.changeMu.Unlock()
 }
 
 // WaitChange blocks until the named field changes from current, then returns