When a worker re-joins and already exists in workermgr, makeWorker_()
now overrides the existing slot instead of returning EEXIST. Admin-set
flags (LOCKEDU, CLOSED, HIDDEN, CLOSED_BY_ADMIN) survive the override.
Add XCE_WORKER_CLOSED_BY_ADMIN flag to distinguish admin-closed hosts
from workload-closed ones. worker_ctrl sets/clears it alongside CLOSED.
Refactor worker_init/worker_destroy into copy_worker/kill_worker helpers
shared by both fresh creation and override paths.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
