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.lock

@@ -1365,6 +1365,8 @@ dependencies = [
  "dirs 5.0.1",
  "g3-config",
  "g3-core",
+ "g3-planner",
+ "g3-providers",
  "hex",
  "indicatif",
  "ratatui",
@@ -1499,6 +1501,18 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "g3-planner"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "const_format",
+ "g3-providers",
+ "serde",
+ "serde_json",
+ "tokio",
+]
+
 [[package]]
 name = "g3-providers"
 version = "0.1.0"