health-probes: clear stale results on start/restart, fast 10s probing until healthy

- Clear HealthProbe on StartStack/RestartStack so stale unhealthy state
  isn't re-applied by RefreshStatus
- Use 10s probe interval for unhealthy/new stacks (nil HealthProbe probes
  immediately on next tick), switch to normal 5m interval once healthy
- Scheduler frequency 1m → 10s to support fast probing

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

controller/cmd/controller/main.go

@@ -222,7 +222,7 @@ func main() {
 	sched.Every("stack-scan", 2*time.Minute, func(ctx context.Context) error {
 		return stackMgr.ScanStacks()
 	})
-	sched.Every("health-probes", 1*time.Minute, func(ctx context.Context) error {
+	sched.Every("health-probes", 10*time.Second, func(ctx context.Context) error {
 		return stackMgr.RunHealthProbes()
 	})