toolkit: the tool program, read_file and write_file

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/toolkit/src/main.rs

@@ -1,4 +1,15 @@
-fn main() {
-    eprintln!("toolkit: not implemented until M3");
-    std::process::exit(2);
+use std::io::Write;
+use std::process::ExitCode;
+
+fn main() -> ExitCode {
+    let mut rest = std::env::args_os().skip(1);
+    let name = match (rest.next(), rest.next()) {
+        (Some(arg), None) => arg.to_str().unwrap_or("").to_string(),
+        _ => String::new(),
+    };
+
+    let outcome = toolkit::run(&name, &mut std::io::stdin().lock());
+    let _ = std::io::stdout().write_all(outcome.stdout.as_bytes());
+    let _ = std::io::stderr().write_all(outcome.stderr.as_bytes());
+    ExitCode::from(outcome.exit.code())
 }