feat(cli): spacesh-cli scaffold — clap tree, one-shot client, command mapping

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/spacesh-cli/src/output.rs

@@ -0,0 +1 @@
+pub async fn run(_c: crate::cli::Cli) -> i32 { 0 }