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>

.gitignore

@@ -6,10 +6,11 @@ runtime/authlib-injector.jar
 # rendered configs (from tooling/render-config.sh)
 drasl/config/config.toml
 dnsmasq/dnsmasq.conf
-# packwiz external-file metadata + index are rendered from *.pw.toml.tmpl
+# packwiz pack files rendered from pack/**/*.tmpl by tooling/render-pack.sh
 # (domain-dependent build output); source of truth is the .tmpl + custom/ jars.
 pack/mods/*.pw.toml
 pack/index.toml
+pack/CustomSkinLoader/CustomSkinLoader.json
 
 # vendored / mirrored binaries
 mirror/