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feat(core/transport): keep one SSH connection per transport

The SSH transport connected before and disconnected after every run(),
get() and put(), so each command paid a key exchange. Measured from a
test harness against a host at 43 ms ping, a command cost about 0.55 s
that way and about 0.175 s over a kept connection, which made cheap
remote checks expensive enough to be left out. Per-command connections
existed to cope with a remote that reboots and to not leave connections
open at exit; a kept connection handles both by connecting again on
next use when the old one is no longer active, and by being closed by
Cijoe.close(), at pytest unconfigure, or by a finalizer as Python exits.
A connect that fails part way, as when authentication is refused, is
closed rather than kept. A 30 s keepalive keeps an idle connection's
NAT and firewall state. wait_for_transport closes the connection before
each probe, since a kept session can outlive a remote that no longer
accepts new ones, and only a fresh connect tells whether it is up or
down.

Assisted-by: Claude Code:claude-opus-5-5
Signed-off-by: Simon A. F. Lund <os@safl.dk>

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