CLI Output Conventions

Conventions for designing AND testing CLI output so it stays parseable and assertable - exit-code policy (0 success, non-zero failure with stable codes per failure mode), `stdout` for primary data / `stderr` for messages, `--json` / `--plain` for machine-readable output, deterministic ordering and timestamps, `NO_COLOR` / TTY-aware color, `-q` / `--verbose` discipline, and stable `--help` / `--version`. Built on the [Command Line Interface Guidelines][clig]. Use when designing or testing a CLI's output - a new flag, command, or error message, writing CLI assertions, or fixing flaky tests caused by non-deterministic output; it is the assertion contract for `bats-testing` a…