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/training.lua

@@ -0,0 +1,19 @@
+-- Neovim Training Wheels - Build better vim habits
+-- Load nix settings if available
+pcall(require, 'nix-settings')
+
+return {
+  'dlond/training.nvim',
+  enabled = vim.g.training_mode_enabled or false, -- Controlled by Nix
+  lazy = false,
+  dir = vim.fn.stdpath('config') .. '/lua/plugins/training', -- Local "plugin"
+  config = function()
+    require('plugins.config.training').setup()
+  end,
+  keys = {
+    { '<leader>tt', function() require('plugins.config.training').toggle() end, desc = '[T]raining [T]oggle' },
+    { '<leader>ts', function() require('plugins.config.training').show_stats() end, desc = '[T]raining [S]tats' },
+    { '<leader>tg', function() require('plugins.config.training').challenge() end, desc = '[T]raining [G]ame' },
+    { '<leader>?', function() require('plugins.config.training').cheatsheet() end, desc = 'Show efficiency cheatsheet' },
+  },
+}