feat(nix): add neovim package output

Create a custom Neovim package that includes the kickstart.nvim
configuration. The package copies init.lua, lua/, and doc/ directories
and loads them at runtime.

This allows users to build and run this Neovim configuration via:
  nix run github:0xWheatyz/kickstart.nvim

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

flake.nix

@@ -10,9 +10,40 @@
     flake-utils.lib.eachDefaultSystem (system:
       let
         pkgs = nixpkgs.legacyPackages.${system};
+
+        # Custom Neovim package with this configuration
+        neovimConfig = pkgs.stdenv.mkDerivation {
+          name = "kickstart-nvim-config";
+          src = self;
+
+          installPhase = ''
+            mkdir -p $out
+            cp -r init.lua $out/
+            cp -r lua $out/
+            cp -r doc $out/
+            ${pkgs.lib.optionalString (builtins.pathExists ./AGENT.md) "cp AGENT.md $out/"}
+          '';
+        };
+
+        neovim-kickstart = pkgs.neovim.override {
+          configure = {
+            customRC = ''
+              lua << EOF
+              -- Set XDG_CONFIG_HOME to point to our config
+              vim.env.NVIM_APPNAME = vim.env.NVIM_APPNAME or "nvim-kickstart"
+
+              -- Load the kickstart configuration
+              dofile("${neovimConfig}/init.lua")
+              EOF
+            '';
+          };
+        };
       in
       {
-        # Outputs will be added in subsequent commits
+        packages = {
+          default = neovim-kickstart;
+          neovim = neovim-kickstart;
+        };
       }
     );
 }