agent: replace sync.Cond with close-channel signal for WaitChange

sync.Cond is the wrong primitive for Go — it requires holding a mutex
during Wait and needs context.AfterFunc hacks to integrate with ctx
cancellation.

Replace with a simple signal channel: notifyChange() closes it and
swaps in a fresh one; WaitChange() selects on the channel and
ctx.Done(). Simpler, idiomatic, no spurious wakeups.

agent/agent.go

@@ -41,10 +41,10 @@ type Agent struct {
 	fifo          Fifo   // prompt queue
 	toolCallCount atomic.Int64
 	pendingInject atomic.Pointer[string]
-	submitMu      sync.Mutex // serializes Submit calls (commands + turns)
-	stateMu       sync.RWMutex
-	changeMu      sync.Mutex
-	changeCond    *sync.Cond
+	submitMu     sync.Mutex // serializes Submit calls (commands + turns)
+	stateMu      sync.RWMutex
+	signalMu     sync.Mutex
+	signalCh     chan struct{} // closed on state change; replaced with a fresh channel
 
 	// Event handler — receives events from this agent.
 	output          EventHandler
@@ -171,43 +171,36 @@ func (ag *Agent) setReply(reply string) {
 
 // notifyChange wakes all goroutines waiting on state changes.
 func (ag *Agent) notifyChange() {
-	ag.changeMu.Lock()
-	ag.changeCond.Broadcast()
-	ag.changeMu.Unlock()
+	ag.signalMu.Lock()
+	close(ag.signalCh)
+	ag.signalCh = make(chan struct{})
+	ag.signalMu.Unlock()
 }
 
 // WaitChange blocks until the agent's state differs from current.
 // Returns the new value and true, or ("", false) if ctx is cancelled.
 func (ag *Agent) WaitChange(ctx context.Context, field, current string) (string, bool) {
-	done := make(chan struct{})
-	context.AfterFunc(ctx, func() {
-		ag.changeMu.Lock()
-		ag.changeCond.Broadcast()
-		ag.changeMu.Unlock()
-		close(done)
-	})
-
-	ag.changeMu.Lock()
 	for {
+		// Snapshot the signal channel before reading state.
+		ag.signalMu.Lock()
+		ch := ag.signalCh
+		ag.signalMu.Unlock()
+
 		var val string
 		switch field {
 		case WatchState:
 			val = ag.State()
 		default:
-			ag.changeMu.Unlock()
 			return "", false
 		}
 		if val != current {
-			ag.changeMu.Unlock()
 			return val, true
 		}
-		if ctx.Err() != nil {
-			ag.changeMu.Unlock()
-			return "", false
-		}
-		ag.changeCond.Wait()
-		if ctx.Err() != nil {
-			ag.changeMu.Unlock()
+		// Wait for either a state change or context cancellation.
+		select {
+		case <-ch:
+			// State changed — loop to re-check.
+		case <-ctx.Done():
 			return "", false
 		}
 	}
@@ -588,7 +581,7 @@ func NewAgent(cfg AgentCfg) *Agent {
 		flush:           cfg.Flush,
 		state:           "idle",
 	}
-	ag.changeCond = sync.NewCond(&ag.changeMu)
+	ag.signalCh = make(chan struct{})
 	ag.wireToolsChanged()
 	return ag
 }