Add make install for global sandboxer CLI

Document uv tool install path and uv run fallback so sandboxer is
available outside the project venv after make install.

Makefile

@@ -1,6 +1,6 @@
 .DEFAULT_GOAL := help
 
-.PHONY: help setup test lint format build check cli-version
+.PHONY: help setup install test lint format build check cli-version
 
 help: ## List available make targets
 	@awk 'BEGIN {FS = ":.*## "}; /^[a-zA-Z0-9_.-]+:.*## / {printf "  %-16s %s\n", $$1, $$2}' $(MAKEFILE_LIST)
@@ -8,6 +8,9 @@ help: ## List available make targets
 setup: ## Sync dependencies into .venv
 	uv sync --all-groups
 
+install: ## Install sandboxer CLI to ~/.local/bin (editable)
+	uv tool install -e . --force
+
 test: ## Run the test suite
 	uv run pytest