Function agent_roster_add_remote 

pub fn agent_roster_add_remote(
    slug: String,
    display_name: String,
    transport_kind: String,
    endpoint: String,
    infer_tool: Option<String>,
    model: Option<String>,
    system_prompt: Option<String>,
) -> Result<(), String>

Convenience: create/update a REMOTE-MCP agent from primitives so the UI never hand-builds the backend enum. transport_kind ∈ "tcp" | "http" | "stdio"; endpoint is host:port / a URL / a command line respectively.