refactor(pack): template custom mod metadata so BASE_DOMAIN isn't frozen

The download URL is the only domain-dependent field in the external-file
metadata. Make mods/*.pw.toml build output rendered from committed
mods/*.pw.toml.tmpl (url uses ${BASE_DOMAIN}); render-config.sh and
add-custom-mod.sh render them + packwiz refresh at deploy/add time.

- pack/.packwizignore: keep *.pw.toml.tmpl out of the index
- gitignore rendered pack/mods/*.pw.toml + pack/index.toml (build output)
- seed a minimal index.toml before refresh (packwiz won't bootstrap one)
- add-custom-mod renders inline (BASE_DOMAIN only) — no coupling to
  dnsmasq's HOST_LAN_IP

Domain changes now just need a re-render, not a metadata rewrite.

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

.gitignore

@@ -6,6 +6,10 @@ 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
+# (domain-dependent build output); source of truth is the .tmpl + custom/ jars.
+pack/mods/*.pw.toml
+pack/index.toml
 
 # vendored / mirrored binaries
 mirror/