Add license key gate, migrate marketing site to separate repo

- install.sh: prompt for license key as first step, validate against
  makeyourown.website/api/validate before any system changes
- setup.sh: secondary key check (format-only if .license-key exists,
  server validation if missing)
- Remove website/ directory — marketing site, keyserver, and deploy
  scripts migrated to the websites repo (git.constantprojects.xyz/
  tankadmin/websites.git) for independent deployment
- Update CLAUDE.md, guide.md, .gitignore to reflect migration

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1,10 +1,10 @@
 # Environment and secrets
 .env
 .credentials
+.license-key
 
-# Docker Compose override (used for marketing site on production VPS only)
+# Docker Compose override (legacy, marketing site moved to websites repo)
 /docker-compose.override.yml
-website/deploy/active-marketing.conf
 
 # Persistent data (created by setup.sh)
 websitebox-data/