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/launchd/com.user.alacritty-theme-sync.plist

@@ -4,28 +4,18 @@
 <dict>
     <key>Label</key>
     <string>com.user.alacritty-theme-sync</string>
-    
     <key>ProgramArguments</key>
     <array>
-        <string>/Users/danny/dotfiles/scripts/sync-alacritty-theme.sh</string>
+        <string>/bin/bash</string>
+        <string>/Users/danny/dotfiles/scripts/alacritty-sync-system-theme.sh</string>
     </array>
-    
     <key>StartInterval</key>
     <integer>30</integer>
-    
     <key>RunAtLoad</key>
     <true/>
-    
     <key>StandardOutPath</key>
     <string>/tmp/alacritty-theme-sync.log</string>
-    
     <key>StandardErrorPath</key>
     <string>/tmp/alacritty-theme-sync-error.log</string>
-    
-    <key>EnvironmentVariables</key>
-    <dict>
-        <key>PATH</key>
-        <string>/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin</string>
-    </dict>
 </dict>
 </plist>