Harden CLI errors and exit codes

[keepithandy]

Goal

Make dev-kit feel dependable from PowerShell, terminal scripts, and future CI jobs by standardizing error messages and exit codes.

Proposed work

• Validate --path before running checks.
• Give friendly messages for missing paths, files, and permissions.
• Use 0 for clean pass, 1 for audit failures, and a separate non-zero code for usage/runtime errors if practical.
• Keep tracebacks hidden unless a debug flag is added later.

Why this matters

This repo is meant to support real project hygiene work. Clear CLI behavior makes it easier to trust in scripts and easier to teach from.

Acceptance checks

• Invalid path does not crash with a raw traceback.
• Audit failure and command usage failure are distinguishable.
• README documents the exit-code behavior.
• Tests cover success, audit failure, and invalid input.