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/.packwizignore

@@ -1,3 +1,3 @@
-# Template sources for external-file metadata — rendered to *.pw.toml by
-# tooling/render-config.sh. packwiz must NOT index the templates themselves.
-*.pw.toml.tmpl
+# Template sources for pack metadata/config — rendered to their real files by
+# tooling/render-pack.sh. packwiz must NOT index the templates themselves.
+*.tmpl