Block dotfile access, remove custom CSS, fix cron npm install

- nginx: deny all requests to hidden files (/.git/config was publicly readable)
- nginx: remove CSS injection and /custom/ static file serving
- cron: install script deps at build time into /opt to avoid ro mount conflict
- docker-compose: widen cron build context for package.json COPY
- Delete unused theme/ghost-guild.css

cron/entrypoint.sh

@@ -1,12 +1,6 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-# Install node dependencies for export script
-cd /app
-if [ -f scripts/package.json ]; then
-  cd scripts && npm install --production && cd ..
-fi
-
 # Configure git for automated commits
 git config --global --add safe.directory /app
 git config --global user.email "wiki-bot@ghostguild.org"