cmdInitProgress used planningDir(cwd), which resolves to root .planning
when no active workstream and no --ws/GSD_WORKSTREAM is set — regardless
of mode:workstream. So /gsd-progress confidently reported a stale root
milestone with no signal it was stale.
Fail safe: when .planning/workstreams/ has workstreams AND no active
workstream is resolved, error with an actionable hint naming the available
workstreams and the --ws / `workstream set` fix. Flat mode (no workstreams
dir) and --ws <name> are unchanged.
Regression in tests/init.test.cjs: errors when workstreams exist but none
active (no stale root report); succeeds with --ws; flat mode unchanged.
Closes#1912
