Add dictionary autocompletion to neovim without spell checking

- Add cmp-dictionary plugin to extraPlugins for English word completion
- Configure dictionary source in nvim-cmp with keyword_length = 2
- Set up dictionary paths pointing to /usr/share/dict/words
- Enable case-insensitive matching for better user experience
- Completion triggers after 2+ characters for performance optimization

This provides English word autocompletion without enabling vim's spell
checking features, keeping the interface clean without visual indicators.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

modules/nvim.nix

@@ -97,6 +97,7 @@
           sources = [
             { name = "buffer"; }     # Words from open buffers
             { name = "path"; }       # File system paths
+            { name = "dictionary"; keyword_length = 2; } # English dictionary words
           ];
           
           mapping = {
@@ -113,6 +114,7 @@
     # Extra plugins that don't have dedicated modules
     extraPlugins = with pkgs.vimPlugins; [
       vim-fugitive
+      cmp-dictionary
     ];
 
     # Keymaps
@@ -221,6 +223,13 @@
         },
       })
 
+      -- Dictionary completion setup
+      require("cmp_dictionary").setup({
+        paths = { "/usr/share/dict/words" },  -- Standard dictionary path
+        exact_length = 2,                     -- Minimum length before completion
+        first_case_insensitive = true,        -- Case insensitive matching
+      })
+
       -- Unicode-safe file operations for macOS
       function open_file_with_system_app()
         local filepath = vim.fn.expand('%:p')