feat!: replace zellij with tuios, default to ghostty + kitty (#349)

* feat!: replace zellij with tuios and switch to ghostty + kitty

Terminal environment overhaul:

- add the tuios flake input (upstream 0.8.5; nixpkgs has 0.8.1) and link
  the checked-in config out-of-store; auto-start tuios in nushell instead
  of zellij
- remove zellij entirely: both modules, config, README and nushell
  completions
- remove foot and alacritty, which lack the Kitty graphics protocol;

home/darwin/README.md

@@ -8,7 +8,7 @@ This directory contains macOS-specific Home Manager configurations for Darwin sy
 
 - **default.nix**: Entry point that imports all Darwin configurations
 - **shell.nix**: Shell configurations and environment settings
-- **terminal.nix**: Sets a 15px font size across Alacritty, Ghostty, and Kitty
+- **terminal.nix**: Sets a 15px font size across Ghostty and Kitty
 - **rime-squirrel.nix**: [Rime Squirrel](https://github.com/rime/squirrel) input method
   configuration
 

home/darwin/terminal.nix

@@ -3,7 +3,6 @@ let
   fontSize = 15;
 in
 {
-  programs.alacritty.settings.font.size = lib.mkForce fontSize;
   programs.ghostty.settings.font-size = lib.mkForce fontSize;
   programs.kitty.font.size = lib.mkForce fontSize;
 }