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.

Cargo.toml

@@ -2,6 +2,7 @@
 members = [
     "crates/g3-cli",
     "crates/g3-core", 
+    "crates/g3-planner",
     "crates/g3-providers",
     "crates/g3-config",
     "crates/g3-execution",