(remote): SWITCHBOARD_SSH_PATH only reaches the tmux attach; the mirror, watch, stop and scp use ssh from PATH

[jbr-sekoia]

SWITCHBOARD_SSH_PATH reads as the ssh binary for remote hosts, but only the attach PTY honours it:

• read: remote-attach.js:230 (defaultResolveSshPath);
• spawn a bare 'ssh' from PATH: the mirror (remote-transport.js:215, :290), the watch channel (remote-watch.js:137) and stop (remote-attach.js:260);
• spawns scp from PATH the same way: remote-transport.js:238.

A user who sets it because ssh is not on PATH, or to use a specific client (a wrapper, a Windows OpenSSH path), gets an attach that works and a mirror that does not.

What should happen

One resolver for the ssh binary, used by every remote spawn, and a matching one for scp (derived from the same directory, or SWITCHBOARD_SCP_PATH).

Acceptance

• Every spawn/execFile of ssh or scp in the remote modules goes through the resolver; a test enumerates them.
• docs/remote-hosts.md and docs/settings.md (from (docs): rewrite the README and docs/ against the code, one page per area #357) describe the variable accordingly.