chore(flake): add nix-index-database input

Prebuilt nix-index database (follows nixpkgs) so command-not-found works
immediately without a manual `nix-index` run. Consumed in shell.nix.
Lock change is purely additive; existing pins are unchanged.

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

flake.nix

@@ -28,6 +28,12 @@
       url = "gitlab:rycee/nur-expressions?dir=pkgs/firefox-addons";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+    # Prebuilt nix-index database so "command not found -> which package
+    # provides it" works immediately (no manual `nix-index` run). See shell.nix.
+    nix-index-database = {
+      url = "github:nix-community/nix-index-database";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
   outputs =