refactor: migrate to template-based theme system, clean up old per-theme configs

- Remove per-theme app configs (alacritty, foot, ghostty, hyprland,
  hyprlock, mako, rofi, swayosd, tmux, walker, waybar, etc.) now
  handled by default/themed/ templates
- Update omarchy scripts (bg-next, list, set) for new template flow
- Update configs: bootstrap, nvim, rofi, tmux, mako
- Tweak installers (02-install-base.sh, btop, desktop, goose, nvim,
  obsidian, theme, tmux)
- Remove obsolete hyprland-rofi-* and hyprland-walker-* scripts
- Update neovim.lua across remaining themes

bootstrap

@@ -1,8 +1,8 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-if [[ -z "${DOTFILE_DIR:-}" ]]; then
-  export DOTFILE_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+if [[ -z "${DOTFILES_DIR:-}" ]]; then
+  export DOTFILES_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
 fi
 
 script_dir="$script_dir/installer"