Theme Helix with the current Omarchy theme (#5530)

Helix had no theme integration. Installing it from the menu just ran
omarchy-pkg-add and left the editor on whatever default theme it shipped
with, regardless of the active Omarchy palette.

This adds `default/themed/helix.toml.tpl` so the template
renderer produces a helix.toml on every theme switch, and
`omarchy-install-helix` that wires Helix up:
  - install the package
  - symlink `~/.config/helix/themes/omarchy.toml` at the rendered theme
  - seed a config.toml selecting that theme when the user has none

The install menu routes Helix through this script. omarchy-restart-helix
joins omarchy-theme-set and signals Helix with SIGUSR1 so theme changes
are live [[1]], the same way as other packages. Arch-based distros
package the binary as 'helix' rather than upstream 'hx', so the new
scripts target 'helix' and the installer ends with a hint about aliasing
'hx'.

[1]: https://docs.helix-editor.com/configuration.html
Ref: https://github.com/basecamp/omarchy/discussions/4986

bin/omarchy-menu

@@ -352,7 +352,7 @@ show_install_editor_menu() {
   *Cursor*) install_and_launch "Cursor" "cursor-bin" "cursor" ;;
   *Zed*) install_and_launch "Zed" "zed" "dev.zed.Zed" ;;
   *Sublime*) install_and_launch "Sublime Text" "sublime-text-4" "sublime_text" ;;
-  *Helix*) install "Helix" "helix" ;;
+  *Helix*) present_terminal omarchy-install-helix ;;
   *Emacs*) install "Emacs" "emacs-wayland" && systemctl --user enable --now emacs.service ;;
   *) show_install_menu ;;
   esac