chore(config): add automated Rust install to make setup

Adds tooling/install-rust script that installs Rust via rustup if
not present, with clippy and rustfmt components. Idempotent — skips
when already installed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Makefile

@@ -33,9 +33,7 @@ setup: setup-rust setup-godot setup-tooling decisions-sync
 	@echo "Dev environment ready."
 
 setup-rust:
-	@echo "Checking Rust toolchain..."
-	@command -v cargo >/dev/null 2>&1 || { echo "Install Rust: https://rustup.rs"; exit 1; }
-	@rustup component add clippy rustfmt
+	@tooling/install-rust
 
 setup-godot:
 	@GODOT_VERSION=$(GODOT_VERSION) tooling/install-godot