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;

flake.nix

@@ -55,7 +55,7 @@
 
     # https://github.com/catppuccin/nix
     # main carries the rust-overlay-style deprecation fix for its vscode
-    # package (nodejs -> nodejs-slim); v26.05 still emits the eval warning.
+    # package (nodejs -> nodejs-slim).
     catppuccin = {
       url = "github:catppuccin/nix/main";
       inputs.nixpkgs.follows = "nixpkgs";
@@ -165,5 +165,12 @@
       url = "github:ryan4yin/pyclipsync/v0.1.3";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+
+    # terminal window manager / multiplexer; nixpkgs lags behind upstream,
+    # so track the upstream flake.
+    tuios = {
+      url = "github:Gaurav-Gosain/tuios";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 }