chore: extract arcade into standalone repo

Arcade lives as its own pnpm workspace at ~/Documents/Code/arcade
now, with no @mana/* coupling. This drops every reference and the
games/ directory from the monorepo.

Removes:
- games/ directory (89 files: web + server + 22 HTML games + screenshots)
- @arcade/web, @arcade/server pnpm workspace entries (games/* globs)
- arcade scripts in root package.json (4 scripts)
- arcade.mana.how from mana-auth trusted origins + CORS_ORIGINS
- arcade entries in mana-apps registry, app-icons, URL overrides
- arcade.mana.how from cloudflared tunnel + prometheus blackbox probes
- arcade-web service block in docker-compose.macmini.yml
- generate-env.mjs entries for arcade server + web
- BRANDING_ONLY 'arcade' entry in registry consistency spec
- dead arcade translation keys in GuestWelcomeModal (DE+EN)
- arcade mention in CLAUDE.md, authentication guideline, MODULE_REGISTRY

Verified:
- services/mana-auth/src/auth/sso-config.spec.ts: 8/8 pass
- pnpm install regenerates lockfile cleanly (-536 lines)
- no remaining 'arcade' refs outside historical snapshot docs

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.claude/guidelines/authentication.md

@@ -112,8 +112,6 @@ app.get('/api/v1/data', (c) => {
 });
 ```
 
-> Note: Arcade's server (`@arcade/server`) does not require auth — game generation and community submission are public endpoints.
-
 ## Environment Variables
 
 ```env