Adds bin/verify-node-ssh.sh: checks reverse-tunnel listeners and SSH
auth for each fleet node port from the VM. Distinguishes dark nodes
(no listener) from auth failures (authorized_keys perms/keys).
Verification 2026-10-09:
- muse/2225, 646/2226, pip/2227, muse-main/2224, opm/2228: LISTEN
- def/2229, dev/2230: DARK (no reverse tunnel)
- All listening nodes reject VM super key (expected: nodes authorize
per-operator/id_frontdoor keys, not the VM super key)
Fixes#211