feat(nix): add devShells output for local testing

Add devShells.default to enable testing the Neovim configuration
locally with 'nix develop'. The shell includes Neovim with the
kickstart configuration and all recommended dependencies.

Usage:
  nix develop    # Enter shell with nvim available
  nix develop -c nvim  # Run nvim directly

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

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

flake.nix

@@ -68,6 +68,22 @@
           default = neovim-kickstart;
           neovim = neovim-kickstart;
         };
+
+        devShells.default = pkgs.mkShell {
+          buildInputs = [
+            neovim-kickstart
+            pkgs.ripgrep
+            pkgs.fd
+            pkgs.gcc
+            pkgs.gnumake
+            pkgs.git
+          ];
+
+          shellHook = ''
+            echo "Neovim kickstart development environment"
+            echo "Run 'nvim' to start Neovim with the kickstart configuration"
+          '';
+        };
       }
     ) // {
       # Home Manager module (not system-specific)