feat: add gx.nvim, comment-box, and machine-local override support

- Add gx.nvim for smart URL opening (GitHub, plugins, package.json, search)
- Add comment-box.nvim with keymaps for creating/deleting comment boxes
- Load machine-local secrets.lua and local.lua from ~/.config/.user-dots/nvim/
- Update README with new plugins, keymaps, and portability highlight

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

init.lua

@@ -34,3 +34,12 @@ require('lazyload')
 require('options')
 require('plugins')
 require('keymaps')
+
+-- Source machine-local and secret overrides (silently ignored if absent)
+local user_dots = vim.fn.expand("~/.config/.user-dots/nvim/")
+for _, file in ipairs({ "secrets.lua", "local.lua" }) do
+  local path = user_dots .. file
+  if vim.fn.filereadable(path) == 1 then
+    dofile(path)
+  end
+end