_read_json_overrides is documented to let env vars outrank the
persisted cli-config.json, but it decided per-alias and broke on the
first alias found in either env or the file. When a multi-alias field
(e.g. api_key via LLM_API_KEY/OPENAI_API_KEY) was set in the env under
one alias but stored in the file under another, the stale file value
was surfaced as an init kwarg and overrode the live env var. A
lowercase env var was also missed (settings use case_sensitive=False).
Decide whether a field is already set in the environment by checking
all of its aliases case-insensitively before consulting the file. Add
regression tests for the cross-alias and case-insensitive cases.
Closes#688
sean-kim05