turn the CLI "model" option to a CLI argument

crates/fj-app/src/args.rs

@@ -10,7 +10,6 @@ use fj_math::Scalar;
 #[command(version = fj::version::VERSION_FULL)]
 pub struct Args {
     /// The model to open
-    #[arg(short, long)]
     pub model: Option<PathBuf>,
 
     /// Export model to this path

tools/export-validator/src/main.rs

@@ -20,7 +20,7 @@ fn main() -> anyhow::Result<()> {
         let exit_status = Command::new("cargo")
             .arg("run")
             .arg("--")
-            .args(["--model", &model])
+            .arg(&model)
             .args(["--export", export_file_path_str])
             .status()?;