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>

pack/mods/justenoughresources-neoforge-1-21-1-1-6-0-17.pw.toml.tmpl

@@ -0,0 +1,8 @@
+name = "JustEnoughResources-NeoForge-1.21.1-1.6.0.17"
+filename = "JustEnoughResources-NeoForge-1.21.1-1.6.0.17.jar"
+side = "both"
+
+[download]
+url = "http://pack.${BASE_DOMAIN}/custom/JustEnoughResources-NeoForge-1.21.1-1.6.0.17.jar"
+hash-format = "sha256"
+hash = "7a47d69b5530704690d5a5f0726cd74a2d6c57df17dd4a8589c4ab5212f91460"