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.

internal/ui/add_form.go

@@ -438,7 +438,12 @@ func (m *addFormModel) renderGeneralTab() string {
 		b.WriteString(fieldStyle.Render(field.label))
 		b.WriteString("\n")
 		b.WriteString(m.inputs[field.index].View())
-		b.WriteString("\n\n")
+		b.WriteString("\n")
+		if field.index == tagsInput && m.focused == tagsInput {
+			b.WriteString(m.styles.FormHelp.Render(`  tip: use "hidden" to hide this host from the list`))
+			b.WriteString("\n")
+		}
+		b.WriteString("\n")
 	}
 
 	return b.String()