fix(cli): remove the --arch flag

KVM runs only the host architecture, so the flag could not mean what
it suggested: a mismatched value downloaded a foreign rootfs that
extracted cleanly and died at boot with a cryptic guest-side exec
error. The flag's real job, picking the image variant to fetch, is
the library's; RunOptions.arch keeps that role and an empty value
still resolves to the host architecture.

crates/ecrou-cli/src/cli.rs

@@ -46,9 +46,6 @@ pub struct RunArgs {
     /// Guest environment KEY=VALUE (repeatable)
     #[arg(short = 'e', long = "env")]
     pub env: Vec<String>,
-    /// Target architecture; the host architecture when omitted
-    #[arg(short = 'a', long)]
-    pub arch: Option<String>,
     /// Suppress console noise
     #[arg(long)]
     pub quiet: bool,

crates/ecrou-cli/src/main.rs

@@ -38,9 +38,6 @@ fn cmd_run(args: &RunArgs, cli_verbose: bool) -> std::process::ExitCode {
         no_cache: args.no_cache,
         ..RunOptions::default()
     };
-    if let Some(arch) = &args.arch {
-        options.arch = arch.clone();
-    }
     for spec in &args.env {
         match parse_env(spec) {
             Ok((k, v)) => options.env.push((k, v)),