chore: scaffold cargo workspace and crate skeletons

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

crates/spacesh-pty/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "spacesh-pty"
+edition.workspace = true
+version.workspace = true
+
+[dependencies]
+portable-pty.workspace = true
+tokio.workspace = true
+bytes.workspace = true
+anyhow.workspace = true

crates/spacesh-pty/src/lib.rs

@@ -0,0 +1 @@
+// PtyHandle implemented in Task 3.