Function build_provider_config 

pub fn build_provider_config(
    cli_config: &CliConfig,
    cli_args: &CliArgs,
) -> AiConfig

Build an AiConfig by merging CLI args, config file, and defaults.

Priority (highest first):

1. CLI argument (--model)
2. Config file (~/.oy-ai-agent/config.toml)
3. Hardcoded default

api_key is required: if none of the sources provide it, the process exits.