_read_main_api_key_if_same_host read base_url and api_key through the
per-field readers, which fall back to config.yaml field by field. A live
main runtime that is keyless or uses key_cmd therefore paired its own
base_url with config.yaml's key, and an auxiliary task pointed at the live
endpoint received the key issued for config.yaml's endpoint.
The anchor and the key now come from one source: the runtime bound for the
turn, else config.yaml. Documents the origin rule (EN + zh-Hans).
(cherry picked from commit ad3de0cf6fd35ee783c5d999ed98e913d23ffcab)
e6ec4aa9ed