When mem0.json exists but is missing the api_key (e.g. after running
`hermes memory setup`), the plugin reports "not available" even though
MEM0_API_KEY is set in .env. This happens because _load_config()
returns the JSON file contents verbatim, never falling back to env vars.
Use env vars as the base config and let mem0.json override individual
keys on top, so both config sources work together.
Fixes: mem0 plugin shows "not available" despite valid MEM0_API_KEY in .env
5e3303b3d8