feat: add channel enable/disable, prefab docs, and UI updates

Add per-channel enable/disable toggle to the in-game logging system
with Enable()/Disable() on InGameLogger and EnableChannel/DisableChannel
on IGameLogStore. Update README with prefab setup guide and enable/disable
documentation. Update character creation and log container prefabs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Packages/com.jovian.ingame-logging/Runtime/InGameLogger.cs

@@ -19,5 +19,17 @@ namespace Jovian.InGameLogging {
         public void Log(string message, string hexColor) {
             store.Add(channel, $"<color={hexColor}>{message}</color>");
         }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void Enable() {
+            store.EnableChannel(channel);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void Disable() {
+            store.DisableChannel(channel);
+        }
+
+        public bool IsEnabled => store.IsChannelEnabled(channel);
     }
 }