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>

flake.nix

@@ -17,9 +17,20 @@
       url = "github:oxalica/rust-overlay";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+
+    # nixvim: declarative Neovim as home-manager options — the whole editor
+    # (plugins, LSP, keymaps) lives in modules/nvim.nix, pinned by this flake's
+    # lock. Track the release branch matching nixpkgs (a mismatched branch fails
+    # with `vimPlugins.<name> not found`). `follows` keeps one nixpkgs: both are
+    # on the same stable channel, so the version skew nixvim warns about (only an
+    # issue against unstable) does not arise here.
+    nixvim = {
+      url = "github:nix-community/nixvim/nixos-26.05";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
-  outputs = inputs@{ self, nixpkgs, home-manager, rust-overlay, ... }:
+  outputs = inputs@{ self, nixpkgs, home-manager, rust-overlay, nixvim, ... }:
     let
       system = "x86_64-linux";
     in {
@@ -28,7 +39,11 @@
       #   imports = [ config.nixosModules.default ];
       #   home-manager.users.<you>.imports = [ config.homeModules.default ];
       nixosModules.default = import ./modules/nixos.nix;
-      homeModules.default = import ./modules/home.nix;
+      # nixvim's home-manager module rides along so `modules/nvim.nix` can set
+      # `programs.nixvim.*`; consumers get the editor by importing this one module.
+      homeModules.default = {
+        imports = [ nixvim.homeModules.nixvim ./modules/home.nix ];
+      };
 
       # ---- The example / template host -----------------------------------
       # Real and buildable; dogfoods the modules above. Clone, replace