feat: add Harpoon and Training mode plugins

- Add Harpoon for quick file navigation
- Add Training mode for learning vim motions without crutches
- Configure keymaps for both plugins
- Add Nix integration for training mode toggle

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

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

lua/core/options.lua

@@ -4,6 +4,9 @@
 vim.g.mapleader = ' '
 vim.g.maplocalleader = ' '
 
+-- Load Nix-controlled settings if available
+pcall(require, 'nix-settings')
+
 -- Place custom vim options here
 
 -- Set based on your font installation