Trim comments and use implicit null checks for Unity objects

Reduce comments to non-obvious "why" notes and convert UnityEngine.Object
null checks to implicit bool (if (obj) / if (!obj)). Input System devices
(Mouse.current, Keyboard.current) and plain arrays keep explicit != null.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/PaintMode.cs

@@ -1,7 +1,6 @@
 /// <summary>
-/// Mode 2: the player stands still and the mesh is painted with the left mouse
-/// button. The camera orbits only while the right mouse button is held, so
-/// dragging with the left button paints instead of rotating the view.
+/// Player stands still, the mesh is painted with the left mouse button. The
+/// camera orbits only while the right mouse button is held.
 /// </summary>
 public class PaintMode : GameMode
 {
@@ -9,13 +8,13 @@ public class PaintMode : GameMode
 
     public override void Enter()
     {
-        if (Manager.Player != null)
+        if (Manager.Player)
             Manager.Player.ControlEnabled = false;
 
-        if (Manager.CameraInput != null)
+        if (Manager.CameraInput)
             Manager.CameraInput.LookMode = ThirdPersonOrbitInput.OrbitLookMode.HoldRightMouseButton;
 
-        if (Manager.Painter != null)
+        if (Manager.Painter)
             Manager.Painter.PaintingEnabled = true;
     }
 }