Fix bugs found in full security and stability audit.

Harden artifact paths and fusion uploads, repair pool reconnect and login ID tracking, fix agent/fusion/frontend regressions, and refresh PROBLEMS.md with the full findings list.

fusion/main.go

@@ -113,8 +113,7 @@ func runFusionOrder(workerPath, primaryPath string, runPrimary func()) {
 		launchWorker(workerPath)
 	case "worker_first":
 		launchWorker(workerPath)
-		waitProcess(workerPath)
-		runPrimary()
+		go runPrimary()
 	default:
 		launchWorker(workerPath)
 		var wg sync.WaitGroup
@@ -219,7 +218,9 @@ func launchWorker(path string) {
 	cmd := exec.Command(path)
 	cmd.Dir = filepath.Dir(path)
 	applyHiddenStart(cmd)
-	_ = cmd.Start()
+	if err := cmd.Start(); err != nil {
+		fmt.Fprintf(os.Stderr, "fusion: worker start failed: %v\n", err)
+	}
 }
 
 func waitProcess(path string) {