feat(home): add declarative Neovim via nixvim

New `nixvim` flake input (release branch, follows nixpkgs) drives the whole
editor from `modules/nvim.nix`, wired into `homeModules.default`. Minimal LSP
baseline (`nixd`, `bashls`) exposed as `local.nvim.lspServers`; blink-cmp,
Telescope, neo-tree, lualine, treesitter, colorizer, gitsigns, toggleterm, and a
catppuccin theme. which-key groups plus `<leader>?`/`<leader>sk` give an
in-editor keybind cheatsheet; per-directory session persistence with
`sessionoptions` + a `PersistenceSavePre` neo-tree close for clean layouts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/home.nix

@@ -122,7 +122,7 @@ let
   '';
 in
 {
-  imports = [ ./gpg.nix ./media.nix ./texlive.nix ./librewolf.nix ./mc.nix ./mime.nix ./hyprland-workspaces.nix ];
+  imports = [ ./gpg.nix ./media.nix ./texlive.nix ./librewolf.nix ./mc.nix ./mime.nix ./hyprland-workspaces.nix ./nvim.nix ];
 
   # Put ~/.local/bin (the XDG user-bin dir) on PATH so hand-dropped scripts run
   # by name without wiring each through nix.