Fix Session 0 issue: spawn screen capture helper in user session

- CaptureModeRunner: captures desktop in user session via schtasks,
  sends JPEG frames over named pipe to service
- RtcService: creates named pipe server, spawns helper via schtasks
  (same pattern as NotificationService), reads frames and forwards to WS
- Program.cs: added --rdp-capture <pipeName> mode
- Multi-monitor: captures all screens combined

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

agent-cs/Program.cs

@@ -19,6 +19,10 @@ internal class Program
                     RunNotification(args.Length > 1 ? args[1] : "");
                     return;
 
+                case "--rdp-capture":
+                    CaptureModeRunner.Run(args.Length > 1 ? args[1] : "");
+                    return;
+
                 case "--dashboard":
                     RunDashboard();
                     return;