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-light.yml

@@ -1,28 +0,0 @@
-# Catppuccin Latte (Light) theme for Alacritty
-[colors.primary]
-background = "0xeff1f5"  # base
-foreground = "0x4c4f69"  # text
-
-[colors.cursor]
-text = "0xeff1f5"  # base
-cursor = "0xdc8a78"  # rosewater
-
-[colors.normal]
-black = "0x5c5f77"  # surface1
-red = "0xd20f39"  # red
-green = "0x40a02b"  # green
-yellow = "0xdf8e1d"  # yellow
-blue = "0x1e40af"  # blue
-magenta = "0xea76cb"  # pink
-cyan = "0x179299"  # teal
-white = "0xacb0be"  # subtext1
-
-[colors.bright]
-black = "0x6c6f85"  # surface2
-red = "0xd20f39"  # red
-green = "0x40a02b"  # green
-yellow = "0xdf8e1d"  # yellow
-blue = "0x1e40af"  # blue
-magenta = "0xea76cb"  # pink
-cyan = "0x179299"  # teal
-white = "0xbcc0cc"  # subtext0