The mem0 plugin previously hardcoded api.mem0.ai as the endpoint.
This adds a `host` config key and MEM0_HOST env var so users can
point the plugin at a self-hosted Mem0 instance.
Changes:
- _load_config(): read MEM0_HOST env var
- is_available(): accept host OR api_key (self-hosted may not need a real key)
- get_config_schema(): add host field
- initialize(): read host from config
- _get_client(): pass host kwarg to MemoryClient when set
- system_prompt_block(): show target (cloud vs URL)
- README: document self-hosted setup
buihongduc132
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