feat(macos): Alacritty follows system light/dark appearance

New setup — due for review after you run darwin-rebuild switch and
live with it for a few days. See CLAUDE.md (Alacritty) and
assets/alacritty/README.md.

- HM: import active-colors.toml + Catppuccin latte/mocha fragments
- nix-darwin: launchd.user.agents.alacritty-system-theme + PATH helper
- fish: background sync on Darwin; theme.sh no longer rebuilds for Alacritty
- Remove switch-alacritty-theme.sh (sed + darwin-rebuild per toggle)

Made-with: Cursor

nixos/fish.nix

@@ -24,6 +24,11 @@
 
       set fish_greeting 🐟: (set_color yellow; date +%T; set_color green; date --iso-8601 2>/dev/null; or date +%F; set_color normal)
 
+      # Alacritty palette follows macOS appearance; refresh when opening a shell (LaunchAgent also polls).
+      if test (uname -s) = Darwin
+        bash ~/dotfiles/scripts/alacritty-sync-system-theme.sh >/dev/null 2>&1 &
+      end
+
       # name: Default
       # author: Lily Ballard
       # edits: DannyDannyDanny