add-cli-argument

# Adding CLI Arguments Client CLI arguments are defined in a shared struct to avoid duplication: 1. **Add to `ClientArgs`** in `modelexpress_common/src/client_config.rs` - Single source of truth for shared arguments - Register the variable name in `modelexpress_common/src/envs.rs` and reference the constant: `#[arg(long, env = crate::envs::MODEL_EXPRESS_...)]` (never a bare string literal, so the CLI and the `envs` getters cannot drift). All env-var names live in `modelexpress_common/src/envs.rs` (Rust) and `modelexpress/envs.py` (Python). - Do NOT use `-v` short flag (reserved for CLI's verbose) 2. **Update `ClientConfig::load()`** in the same file - Add override logic in the "APPLY CLI ARGUMENT OVERRIDES" section 3. **Do NOT duplicate in `Cli`** (`modelexpress_client/src/bin/modules/args.rs`) - `Cli` embeds `ClientArgs` via `#[command(flatten)]` - Only add CLI-specific arguments there (e.g., `--format`, `--verbose`) 4. **Add tests** in the `tests` module of `client_config.rs`