add code exploration fast start

This tries to short-circuit multiple round-trips to llm for reading code.
It's a precursor to trying to context engineer tailored to specific tasks.
In initial experiments, it's only marginally faster than regular mode, and burns more tokens.

crates/g3-cli/src/simple_output.rs

@@ -1,4 +1,5 @@
 /// Simple output helper for printing messages
+#[derive(Clone)]
 pub struct SimpleOutput {
     machine_mode: bool,
 }