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

scripts/alacritty-sync-system-theme.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+# Keep Alacritty in sync with macOS light/dark appearance.
+# No Nix rebuild: copies a palette into active-colors.toml; Alacritty reloads via live_config_reload.
+
+set -euo pipefail
+
+[[ "$(uname -s)" == "Darwin" ]] || exit 0
+
+XDG_CONFIG_HOME="${XDG_CONFIG_HOME:-$HOME/.config}"
+ALACRITTY_DIR="$XDG_CONFIG_HOME/alacritty"
+ACTIVE="$ALACRITTY_DIR/active-colors.toml"
+MARKER="$ALACRITTY_DIR/.last-system-theme"
+
+LIGHT="$ALACRITTY_DIR/catppuccin-latte-colors.toml"
+DARK="$ALACRITTY_DIR/catppuccin-mocha-colors.toml"
+
+if [[ ! -f "$LIGHT" || ! -f "$DARK" ]]; then
+  echo "alacritty-sync-system-theme: missing $LIGHT or $DARK (run home-manager switch first)" >&2
+  exit 1
+fi
+
+appearance="$(defaults read -g AppleInterfaceStyle 2>/dev/null || true)"
+if [[ "$appearance" == "Dark" ]]; then
+  want="dark"
+else
+  want="light"
+fi
+
+if [[ -f "$MARKER" ]] && [[ "$(tr -d '\n' <"$MARKER")" == "$want" ]]; then
+  exit 0
+fi
+
+mkdir -p "$ALACRITTY_DIR"
+printf '%s' "$want" >"$MARKER"
+
+if [[ "$want" == "light" ]]; then
+  cp "$LIGHT" "$ACTIVE"
+else
+  cp "$DARK" "$ACTIVE"
+fi