feat: add hidden tag to hide hosts from TUI and search

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.

cmd/root.go

@@ -75,6 +75,8 @@ Examples:
 			return nil, cobra.ShellCompDirectiveError
 		}
 
+		hosts = config.FilterVisibleHosts(hosts)
+
 		var completions []string
 		toCompleteLower := strings.ToLower(toComplete)
 		for _, host := range hosts {