chore(flake): add nixvim input

Declarative Neovim, on the nixos-26.05 release branch and following our
nixpkgs to keep a single nixpkgs in the closure. Consumed by
lyrathorpe/home/editor.nix.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

flake.nix

@@ -46,6 +46,14 @@
       url = "github:cachix/git-hooks.nix";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+    # Declarative Neovim (the editor; see lyrathorpe/home/editor.nix). Release
+    # branch matched to the pinned nixpkgs (26.05); follows our nixpkgs to keep a
+    # single nixpkgs in the closure. editor.nix sets programs.nixvim.nixpkgs.source
+    # to this same input so the home module doesn't warn about the pin.
+    nixvim = {
+      url = "github:nix-community/nixvim/nixos-26.05";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
   outputs =