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-install-helix

@@ -0,0 +1,25 @@
+#!/bin/bash
+
+# Install Helix and configure it to use the current Omarchy theme.
+
+echo "Installing Helix..."
+omarchy-pkg-add helix
+
+mkdir -p ~/.config/helix/themes
+
+# Symlink the rendered Omarchy theme so Helix tracks the active theme
+ln -sf ~/.config/omarchy/current/theme/helix.toml ~/.config/helix/themes/omarchy.toml
+
+# Only seed a config.toml if the user does not already have one
+if [[ ! -f ~/.config/helix/config.toml ]]; then
+  cat >~/.config/helix/config.toml <<'EOF'
+theme = "omarchy"
+EOF
+fi
+
+# Ensure the symlink target exists for users whose current theme predates this template
+if [[ ! -e ~/.config/omarchy/current/theme/helix.toml ]]; then
+  omarchy-theme-refresh
+fi
+
+echo -e "\nArch-based distros ship Helix as 'helix' rather than the upstream 'hx'. Alias 'hx' to 'helix' in your shell configuration if you prefer the shorter command."