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

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

Cargo.toml

@@ -5,6 +5,7 @@ members = [
     "crates/spacesh-pty",
     "crates/spacesh-core",
     "crates/spaceshd",
+    "crates/spacesh-cli",
 ]
 
 [workspace.package]
@@ -25,3 +26,5 @@ portable-pty = "0.8"
 alacritty_terminal = "0.25"
 fs2 = "0.4"
 dirs = "5"
+clap = { version = "4", features = ["derive"] }
+clap_complete = "4"