fix(ci): use fixed staging domain

[yuyufei101]

Summary

• remove the staging releaseName workflow input
• fix the Helm release, factory identifier, and hostname to keystone-staging
• document the single keystone-staging.archebase.cn endpoint
• reserve production releaseName=staging operationally to prevent hostname collision

Validation

• bash .github/scripts/resolve-staging-deploy_test.sh
• bash .github/scripts/validate-staging-cluster_test.sh
• actionlint (custom runner label warning ignored)
• make helm-lint
• git diff --check

Review evidence

• $review-update-loop: 3 rounds on the complete origin/main-v2...HEAD snapshot
• Final reviewer: ACCEPTED / No required findings
• Final updater: no additional changes; all required verification passed

Staging only; production implementation is unchanged.