The previous wording implied external gateway routing was not yet possible.
HERMES_WEBUI_CHAT_BACKEND=gateway already ships; updated wording distinguishes
it from full agent-loop delegation, which is not yet shipped and tracked in #1925.
HERMES_API_URL is only read by the Tasks/cron health probe (api/agent_health.py)
and does not route chat. Adds a plain-language note to Quick Start with the two
supported options for users who run an external endpoint. Closes#3873.
sr-dotcom