When --default <value> is passed, config-get returns the default value
(exit 0) instead of erroring (exit 1) when the key is absent or
config.json doesn't exist. When the key IS present, --default is
ignored and the real value returned.
This lets workflows express optional config reads without defensive
`2>/dev/null || true` boilerplate that obscures intent and is fragile
under `set -e`.
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
