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/plugins/spec/harpoon.lua

@@ -0,0 +1,10 @@
+-- Harpoon - Quick file navigation
+return {
+  'ThePrimeagen/harpoon',
+  branch = 'harpoon2',
+  dependencies = { 'nvim-lua/plenary.nvim' },
+  event = 'VeryLazy',
+  config = function()
+    require('plugins.config.harpoon').setup()
+  end,
+}