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

assets/alacritty/catppuccin-mocha-colors.toml

@@ -0,0 +1,29 @@
+# Catppuccin Mocha — imported by main alacritty.toml; swapped by sync script.
+
+[colors.primary]
+background = "#1e1e2e"
+foreground = "#cdd6f4"
+
+[colors.cursor]
+text = "#1e1e2e"
+cursor = "#f5e0dc"
+
+[colors.normal]
+black = "#45475a"
+red = "#f38ba8"
+green = "#a6e3a1"
+yellow = "#f9e2af"
+blue = "#89b4fa"
+magenta = "#f5c2e7"
+cyan = "#94e2d5"
+white = "#bac2de"
+
+[colors.bright]
+black = "#585b70"
+red = "#f38ba8"
+green = "#a6e3a1"
+yellow = "#f9e2af"
+blue = "#89b4fa"
+magenta = "#f5c2e7"
+cyan = "#94e2d5"
+white = "#a6adc8"