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-dark.toml

@@ -0,0 +1,28 @@
+# Catppuccin Mocha (Dark) theme for Alacritty
+[colors.primary]
+background = "0x1e1e2e"  # base
+foreground = "0xcdd6f4"  # text
+
+[colors.cursor]
+text = "0x1e1e2e"  # base
+cursor = "0xf5e0dc"  # rosewater
+
+[colors.normal]
+black = "0x45475a"  # surface1
+red = "0xf38ba8"  # red
+green = "0xa6e3a1"  # green
+yellow = "0xf9e2af"  # yellow
+blue = "0x89b4fa"  # blue
+magenta = "0xf5c2e7"  # pink
+cyan = "0x94e2d5"  # teal
+white = "0xbac2de"  # subtext1
+
+[colors.bright]
+black = "0x585b70"  # surface2
+red = "0xf38ba8"  # red
+green = "0xa6e3a1"  # green
+yellow = "0xf9e2af"  # yellow
+blue = "0x89b4fa"  # blue
+magenta = "0xf5c2e7"  # pink
+cyan = "0x94e2d5"  # teal
+white = "0xa6adc8"  # subtext0