Make command-line errors actionable and stable

Given [CLI COMMANDS], [FAILURE CASES] and [AUTOMATION USERS], define an error contract with exit code, short message, remediation hint and machine-readable output for each case. Distinguish invalid input, unavailable dependency, permission failure and internal defect without leaking credentials. Provide tests that assert exit codes and safe stderr content. Do not claim a retry is safe unless operations are idempotent. Check that success never prints an error, help text names the right flag, and scripts can distinguish temporary from permanent failure.