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,