fix(exp23): invisible barriers + single-instance guard

- generated_road.unity + generated_track.unity: showBarrierMeshes 1→0.
  Visible barrier meshes would appear in the camera observation and let the
  policy learn from an artificial visual cue that won't exist at eval time.
- exp23: add PID-file guard — aborts immediately if another instance is
  already running, preventing multiple cars from spawning in the sim.

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

agent/experiments/exp23_generated_road_clean.py

@@ -21,6 +21,19 @@ from datetime import datetime
 
 sys.path.insert(0, '/home/paulh/projects/donkeycar-rl-autoresearch/agent')
 
+# Abort if another instance of this experiment is already running.
+_PIDFILE = '/home/paulh/projects/donkeycar-rl-autoresearch/agent/models/exp23-generated-road-clean/current.pid'
+if os.path.exists(_PIDFILE):
+    try:
+        _old = int(open(_PIDFILE).read().strip())
+        if _old != os.getpid():
+            import signal
+            os.kill(_old, 0)        # raises OSError if process is gone
+            print(f'[exp23] Another instance already running (PID {_old}). Exiting.', flush=True)
+            sys.exit(1)
+    except (OSError, ValueError):
+        pass  # stale pid file — safe to continue
+
 import gymnasium as gym
 import numpy as np
 from stable_baselines3 import PPO