refactor(pack): host custom jars from ./custom outside the pack root

Move pack/custom/ -> ./custom/ and bind-mount it into caddy at
/srv/pack/custom (served URL pack.${BASE_DOMAIN}/custom/ unchanged).
Keeping jars outside the packwiz pack root stops `packwiz refresh`
from indexing them as direct files on top of the mods/*.pw.toml
external refs (drops 9 bogus custom/*.jar entries from index.toml).

add-custom-mod.sh now writes jars to ./custom/. Trim the blessingskin
caddy override to just the conf swap (volumes merge by target).

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 = "64c69b2e38fcf389a3de38d240aa95e8309ac60e717c1fe994aef2c4a2684921"
+hash = "2707ae674c7c75371b97cc378ca72abc36bbb513b7db238db111f88a13aa6039"
 
 [versions]
 minecraft = "1.21.1"