refactor(pack): template CustomSkinLoader + unify pack rendering

Add tooling/render-pack.sh: renders all pack/**/*.tmpl with ${BASE_DOMAIN}
(no dnsmasq HOST_LAN_IP coupling) and rebuilds the packwiz index. Both
render-config.sh and add-custom-mod.sh now delegate to it.

CustomSkinLoader.json's skin API roots are no longer frozen — it becomes
CustomSkinLoader.json.tmpl, rendered like the mod metadata. .packwizignore
broadened to *.tmpl; rendered .json gitignored as build output.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pack/CustomSkinLoader/CustomSkinLoader.json.tmpl

@@ -0,0 +1,11 @@
+{
+    "enable": true,
+    "loadlist": [
+        {
+            "name": "Ulicraft Skins",
+            "type": "MojangAPI",
+            "apiRoot": "https://auth.${BASE_DOMAIN}/account/",
+            "sessionRoot": "https://auth.${BASE_DOMAIN}/session/"
+        }
+    ]
+}