chore(github): generic example.com in docs, remove mirror scripts

Replace sac.kalinamall.ru with sac.example.com in public docs/deploy.
Remove Push-Mirror and Rewrite-GitHostUrls scripts from repo.
Simplify workspace-three-repos.md for GitHub-only workflow.

Co-authored-by: Cursor <cursoragent@cursor.com>

deploy/README.md

@@ -5,7 +5,7 @@
 1. [docs/install-ubuntu-24.04-native.md](../docs/install-ubuntu-24.04-native.md)
 2. `deploy/env.native.example` → `/opt/security-alert-center/config/sac-api.env`
 3. `deploy/systemd/sac-api.service` → `/etc/systemd/system/`
-4. `deploy/nginx/sac.conf.tls.example` → `/etc/nginx/sites-available/sac` (wildcard TLS; только `sac.kalinamall.ru`, чужие Host → 444)  
+4. `deploy/nginx/sac.conf.tls.example` → `/etc/nginx/sites-available/sac` (wildcard TLS; только `sac.example.com`, чужие Host → 444)  
    или `sac.conf.example` — только HTTP / отладка
 5. `deploy/sac-deploy.sh` → `/opt/sac-deploy.sh` — полное обновление после `git pull`