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/sac-deploy.sh

@@ -40,7 +40,7 @@ else
  if [ -z "${UPSTREAM}" ]; then
   UPSTREAM="origin/main"
  fi
- log "WARN: fast-forward невозможен (часто после force-push на kalinamall) — reset --hard ${UPSTREAM}"
+ log "WARN: fast-forward невозможен (часто после force-push на upstream) — reset --hard ${UPSTREAM}"
  sudo -u "${APP_USER}" git -C "${APP_ROOT}" reset --hard "${UPSTREAM}"
 fi