Configure Neovim via nixvim with Nix-managed plugins

Add an nvim Module that configures Neovim declaratively through nixvim,
wired as a flake input and consumed as its home-manager module. Options,
globals, keymaps, and plugin settings are typed Nix; the colorscheme
call and two autocmds live in modules/nvim/config.lua via extraConfigLua.
Plugins come from nixpkgs (no plugin manager, no runtime cloning); git,
ripgrep, and fd are provided from Nix; treesitter grammars are built by
Nix so no runtime compiler is needed.

Functionally matches the previous config (plugins, keymaps, options, the
nord colorscheme, markdown conceal, the Neogit blame toggle), verified
headless against the generated init.

hosts/neogaia/default.nix

@@ -42,6 +42,9 @@
   # tmux as the terminal multiplexer.
   modules.tmux.enable = true;
 
+  # Neovim, configured declaratively via nixvim.
+  modules.nvim.enable = true;
+
   # Locale preferences for the base system.
   time.timeZone = "America/New_York";
   i18n.defaultLocale = "en_GB.UTF-8";