usage() hard-coded exit 1, so every --help/-h invocation on issue-assign, issue-create, issue-list, milestone-create, pr-create, pr-list, and pr-merge exited nonzero, logging phantom isError records across fleet lanes and breaking set -e composition. usage() now takes an exit-code arg (default 1); the -h|--help case calls usage 0. All other error paths are untouched. Closes #701