feat(neovim): replace nixvim with programs.neovim (#336)

nixvim has been disabled for a long time (programs.nixvim.enable = false), so
drop the flake input (and prune flake.lock). Port the settings it carried to
Home Manager's programs.neovim: opts and per-plugin setup become an initLua
block plus a plugin list. The catppuccin module already provides the
colorscheme, vi stays unmapped, and the plain system neovim package is
untouched so sudo/nvim --clean keep using it.

home/base/core/editors/neovim/default.nix

@@ -1,105 +1,116 @@
 {
-  config,
-  nixvim,
   pkgs,
   ...
 }:
+let
+  # Parsers are bundled into the plugin, so no runtime `:TSInstall` is needed.
+  # Add a language here to get its treesitter highlighting and queries.
+  treesitter = pkgs.vimPlugins.nvim-treesitter.withPlugins (
+    p: with p; [
+      bash
+      go
+      json
+      lua
+      make
+      markdown
+      nix
+      nu
+      python
+      regex
+      rust
+      toml
+      vim
+      vimdoc
+      xml
+      yaml
+    ]
+  );
+in
 {
+  # Neovim config, ported from the old `programs.nixvim` block. The colorscheme
+  # itself comes from `home/base/core/theme.nix` (catppuccin.nvim is enabled
+  # there and appends its own plugin + setup).
+  #
+  # The plain system package (modules/base/packages.nix) stays in place for
+  # privileged edits: `sudo` uses the system PATH, so
+  # `$SUDO_EDITOR = "nvim --clean"` keeps this user config out of root-owned
+  # files.
+  programs.neovim = {
+    enable = true;
+    vimAlias = true;
 
-  imports = [ nixvim.homeModules.nixvim ];
+    # No python/ruby plugins are configured; use the new explicit defaults
+    # instead of the legacy providers (and their deprecation warnings).
+    withPython3 = false;
+    withRuby = false;
 
-  programs.nixvim.nixpkgs.useGlobalPackages = true;
-  home.shellAliases = {
+    # Language servers started by `vim.lsp.enable` below; each must be on $PATH.
+    extraPackages = with pkgs; [
+      bash-language-server
+      gopls
+      nixd
+      pyright
+      rust-analyzer
+    ];
 
-    vi = "nvim";
-    vim = "nvim";
-  };
+    plugins = [
+      pkgs.vimPlugins.neo-tree-nvim
+      pkgs.vimPlugins.nvim-lspconfig
+      pkgs.vimPlugins.nvim-web-devicons
+      treesitter
+    ];
 
-  programs.nixvim = {
-    # temporarily disabled: nixvim's manpage pulls nixos-render-docs from nixpkgs,
-    # which is broken (patch fails to apply) on the current nixpkgs pin
-    enable = false;
+    initLua = ''
+      -- Editor options (previously `programs.nixvim.opts`).
+      local opt = vim.opt
+      opt.number = true
+      opt.relativenumber = true
+      opt.cursorline = true
+      opt.signcolumn = "auto"
+      opt.clipboard = "unnamedplus"
+      opt.scrolloff = 8
+      opt.swapfile = false
+      opt.title = true
+      opt.titlelen = 20
+      opt.smartindent = false
+      opt.mouse = "a"
+      opt.undofile = true
+      opt.ignorecase = true
+      opt.smartcase = true
+      opt.splitbelow = true
+      opt.splitright = true
+      opt.updatetime = 300
+      opt.wrap = true
+      opt.linebreak = true
 
-    clipboard.providers.wl-copy.enable = pkgs.stdenv.hostPlatform.isLinux;
+      -- Treesitter highlighting; parsers come from the bundled plugin.
+      -- `pcall` skips filetypes that have no parser available.
+      vim.api.nvim_create_autocmd("FileType", {
+        callback = function()
+          pcall(vim.treesitter.start)
+        end,
+      })
 
-    opts = {
-      number = true;
-      relativenumber = true;
-      cursorline = true;
-      signcolumn = "auto";
-      clipboard = "unnamedplus";
-      scrolloff = 8;
-      swapfile = false;
-      title = true;
-      titlelen = 20;
-      smartindent = false;
-      mouse = "a";
-      undofile = true;
-      ignorecase = true;
-      smartcase = true;
-      splitbelow = true;
-      splitright = true;
-      updatetime = 300;
-      wrap = true;
-      linebreak = true;
-    };
+      -- LSP: nvim-lspconfig ships the server definitions, Neovim starts them.
+      vim.lsp.enable({ "nixd", "rust_analyzer", "gopls", "pyright", "bashls" })
 
-    colorschemes.catppuccin = {
-      enable = true;
-      settings = {
-        transparent_background = true;
-      };
-    };
-
-    plugins.lsp.servers = {
-      nixd.enable = true;
-      "rust_analyzer" = {
-        enable = true;
-        installCargo = false;
-        installRustc = false;
-      };
-      gopls.enable = true;
-      pyright.enable = true;
-      bashls.enable = true;
-    };
-
-    plugins.treesitter = {
-      enable = true;
-      grammarPackages = with config.programs.nixvim.plugins.treesitter.package.builtGrammars; [
-        bash
-        json
-        lua
-        make
-        markdown
-        nix
-        regex
-        rust
-        go
-        python
-        toml
-        vim
-        vimdoc
-        xml
-        yaml
-        nu
-      ];
-    };
-
-    plugins.neo-tree = {
-      enable = true;
-      settings = {
+      -- File explorer.
+      require("neo-tree").setup({
         filesystem = {
           filtered_items = {
-            visible = true;
-            hide_dotfiles = false;
-            hide_gitignored = false;
-          };
+            visible = true,
+            hide_dotfiles = false,
+            hide_gitignored = false,
+          },
           follow_current_file = {
-            enabled = true;
-            leave_dirs_open = false;
-          };
-        };
-      };
-    };
+            enabled = true,
+            leave_dirs_open = false,
+          },
+        },
+      })
+    '';
   };
+
+  # Matches the old `programs.nixvim.colorschemes.catppuccin.settings`.
+  catppuccin.nvim.settings.transparent_background = true;
 }