Add Cargo workspace, crate skeletons and the gate

Implemented-By: Laguna S 2.1 (OpenCode)

crates/toolkit/src/lib.rs

@@ -0,0 +1 @@
+//! Entry points that run inside tool containers.

crates/toolkit/src/main.rs

@@ -0,0 +1,4 @@
+fn main() {
+    eprintln!("toolkit: not implemented until M3");
+    std::process::exit(2);
+}