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-latte-colors.toml

@@ -0,0 +1,29 @@
+# Catppuccin Latte — imported by main alacritty.toml; swapped by sync script.
+
+[colors.primary]
+background = "#eff1f5"
+foreground = "#4c4f69"
+
+[colors.cursor]
+text = "#eff1f5"
+cursor = "#dc8a78"
+
+[colors.normal]
+black = "#5c5f77"
+red = "#d20f39"
+green = "#40a02b"
+yellow = "#df8e1d"
+blue = "#1e40af"
+magenta = "#ea76cb"
+cyan = "#179299"
+white = "#acb0be"
+
+[colors.bright]
+black = "#6c6f85"
+red = "#d20f39"
+green = "#40a02b"
+yellow = "#df8e1d"
+blue = "#1e40af"
+magenta = "#ea76cb"
+cyan = "#179299"
+white = "#bcc0cc"