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>

nvim-pack-lock.json

@@ -12,10 +12,18 @@
       "rev": "426dbebe06b5c69fd846ceb17b42e12f890aedf1",
       "src": "https://github.com/catppuccin/nvim"
     },
+    "comment-box": {
+      "rev": "06bb771690bc9df0763d14769b779062d8f12bc5",
+      "src": "https://github.com/LudoPinelli/comment-box.nvim"
+    },
     "conform": {
       "rev": "086a40dc7ed8242c03be9f47fbcee68699cc2395",
       "src": "https://github.com/stevearc/conform.nvim"
     },
+    "cord.nvim": {
+      "rev": "0be090969e978f7442870ef227e09eff7bf3480d",
+      "src": "https://github.com/vyfor/cord.nvim"
+    },
     "flash": {
       "rev": "fcea7ff883235d9024dc41e638f164a450c14ca2",
       "src": "https://github.com/folke/flash.nvim"
@@ -36,6 +44,10 @@
       "rev": "21604255d0e8f9968322f61f2b6c09e5efe1285a",
       "src": "https://github.com/MagicDuck/grug-far.nvim"
     },
+    "gx": {
+      "rev": "ba9c408fc0130fc4548760c3933a81b58fc50de8",
+      "src": "https://github.com/chrishrb/gx.nvim"
+    },
     "inc-rename": {
       "rev": "0074b551a17338ccdcd299bd86687cc651bcb33d",
       "src": "https://github.com/smjonas/inc-rename.nvim"