SSH ControlMaster multiplexing hits sshd MaxSessions=10 and fails, not falls back

Routing many short ssh execs plus long-lived channels (file servers, helpers, language servers) through one ControlMaster to speed them up has a hard limit. sshd's default MaxSessions is 10 per connection, and the 10th session gets 'mux_client_request_session: session request failed: Session open refused by peer' (exit 255). With ControlMaster=no plus a ControlPath, a refused session does not fall back to a direct connection; it just fails. If several apps derive the same ControlPath from a host ID, they share one master and exhaust it together, which breaks reconnects in all of them. Fixes: a separate ops master (its own ControlPath, ControlMaster=auto) for short execs, own connections for long-lived channels, and a per-host concurrency cap well under 10.