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/Cargo.toml

@@ -7,6 +7,8 @@ description = "CLI interface for G3 AI coding agent"
 [dependencies]
 g3-core = { path = "../g3-core" }
 g3-config = { path = "../g3-config" }
+g3-planner = { path = "../g3-planner" }
+g3-providers = { path = "../g3-providers" }
 clap = { workspace = true }
 tokio = { workspace = true }
 anyhow = { workspace = true }