v0.21.2-beta — fix /agent/admin 500 (no _t in views)

Fixed

• /agent/admin 500 — $this->_t() has no view helper in tiger-core; the agent settings screen + aside now pull the registered translator off the registry (Zend_Registry::get('Zend_Translate')->translate()). The unauthenticated smoke only hit the login 302, which had hidden this.
• Corrected the AGENTS.md i18n guidance (views have no _t helper; only Tiger_Form).