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/pack.toml

@@ -6,7 +6,7 @@ pack-format = "packwiz:1.1.0"
 [index]
 file = "index.toml"
 hash-format = "sha256"
-hash = "36d9f52476a127af5837a65e69d25dc1cd9f7e131eb5a76166378351bf40a009"
+hash = "9bca0b1a4434050835cab46116d7c14aafec971ff5a95a1ee0c4232c8bcbc355"
 
 [versions]
 minecraft = "1.21.1"