Hosts tagged with "hidden" are excluded from the TUI list, shell
completions, and sshm search. Direct connections via sshm <host>
still work regardless of the tag.
A toggle key (H) shows or hides hidden hosts in the TUI, with a
yellow banner indicating the active state. The key is documented
in the help panel (h).
A contextual hint on the Tags field in the add and edit forms
reminds the user that "hidden" hides the host from the list.
* Add base for ProxyCommand
* Fix crashes with ProxyCommand
* Add ProxyCommand to README
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Co-authored-by: Simon Gaufreteau <sgaufret@amazon.lu>