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>

docker-compose.blessingskin.yml

@@ -26,12 +26,12 @@
 
 services:
   caddy:
-    # Swap the core ingress conf for the Blessing Skin one (same mount target,
-    # so it shadows the base 00-core.caddy). volumes is a full replacement here.
+    # Swap the core ingress conf for the Blessing Skin one. Compose MERGES
+    # volumes by container path, so mounting the BSS conf at the SAME target
+    # (/etc/caddy/conf.d/00-core.caddy) shadows the base 00-core.caddy; all
+    # other base mounts (pack, custom, static, ca) are kept automatically.
     volumes:
-      - ./caddy/Caddyfile:/etc/caddy/Caddyfile:ro
       - ./caddy/conf.d/00-core-blessingskin.caddy:/etc/caddy/conf.d/00-core.caddy:ro
-      - ./pack:/srv/pack:ro
     depends_on:
       - blessing-skin